Vostro 5620, WD22TB4 dock, flickering screen

Im not sure if this will solve the issue, but as a trouble shooting routine you might wanna try this:
You may have several USB-C ports on your laptop, but only one of them is a gen 2 port. I recon you connect to the dock with the 2. gen USB 3.2 Type-C-port with DisplayPort and powerdelivery?
I'd try to power off both dock and laptop, and power up laptop on battery power - then power up the dock and insert the USB-C from the dock into your laptops only USB-C port with powerdelivery. If you choose the right port, the laptop should be charging.
Now - update all drivers to last version while the dock is connected to this port. This is important as you may not get these drivers installed right unless you are connected to the dock with the correct port.
This might help - if not Id try to power off both, use the common poweradapter charger from dell for powerdelivery to the laptop, and connect the thunderbolt dock to the gen 2 USB-c port. If this don't work, try powering off all, reboot using your poweradapter and try one of the other USB-C ports, just to see if you get lucky with the gen 1 ports.

I appreciate it, but I only have 1 usbc port on the laptop. Dell tech support recommended re-installing the OS, which I did, but still no luck. 

Ok, I see. Is the screen flickering on your laptop, or on an external display connected to the dock? If its flickering on your laptop, it sounds to me like its something wrong with the dock itself, and ill return it for a new. If its on an external display connected to the dock, I did once fix flickering on my external 4k display connected to my dc19 dock by changing from mini dp cable to a full size dp 1.4 cable.

The monitors (external displays) are flickering. I have tried an HDMI cable and an HDMI to display cable instead of 2 displays cables. Any cable, any adapter. We have several of these docks, the laptop does it on all of them. 

 Similar to this issue, which there was no resolution for. 

Dell Precision 5530 i9 + WD19 TB dock + 2 Displayports = screen flickering - Dell Community

Okay, just to rule out the cables completly, I 'm guessing these TB screens do not flicker when you put the HDMI to DP cable directly in your Vostro's HDMI port? In case these are 4k monitors,
off the top of my head, 4k monitors needs at least DP 1.2 or HDMI 2.0 compatible cables to work. 
Anyways, to me, seems like these TB docks are buggy. Both WD19TB and WD22TB4. Read a lot about them on the web, and they are a pain. I was able to upgrade my WD19 dock to WD19DC by changing the dock interface, coz my upgrade from 65W power to 210W Precision 7750 demanded it. The documentation did suggest upgrade from WD19 to WD19TB, but due to bad press I stayed clear, and went for the optional WD19DC instead. The interface was exchanged by simply loosening 2 screws and It worked instantly. I did check for similar options for converting the WD22TB4 into something like WD22DC, but seems like you're regrettably stuck with some sort of TB interface either way. So, to me, looks like you can only change between two TB interfaces w/different number of TB ports(!) Perhaps a WD19 / WD19DC at Ebay would save you some heart ache. I think you might be good with the WD19DC(210W power delivery) or the WD19 (130W powerdelivery) docks, they both have two DP 1.2 ports and deliver over 65W, and I guess that's all you need up to 4k. But say your monitors demands DP 1.4/TB 4 due to 5k or 6k resolution monitors, you need the WD19DCS, that packs two DP 1.4 ports. Due to some shady compatible issues, I do wonder if the best thing to do is to toss the Vostro either way, and opt for powerhouse Precision 7670 or 7770.

A heads up on the WD19DCS and 5k / 6k compability. Somethings off with the config on that dock, the interface cable is USB-C 3.2. If its a gen 2 it might work for DP 1.4 5k/6k, but I'm not convinced. If you're at 5k/6k displays, in my mind, your best bet would be to keep the dock and buy a Precision instead. 

I don't have the issue when I use a WD19. Unfortunately, I don't have any more of those. Maybe I can convince Dell to swap me. Thanks a lot.

The issue appears to be related to TB docks. We've had three users with this issue and each issue stopped when we replaced their dock with the older WD19 or WS19s docking station. The WD19TBS and WD22TB4 docks are both having this issue even after updating to the latest firmware. 

I have very similar issue. New Dell Precision 3571 and the Thunderbolt Dock WD22TB4.

2 external monitors, both Full HD connected to dock station via HMID and DP.

The one on DP get flicker every few minutes.

I tried different tricks, sometimes connecting to another DP socket help, but problem returns after a while.

Looking forward if somebody find the solution

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I've been having an issue where my 38" monitor goes blank for about 30 seconds before coming back on. It happens 3-4 times a day. Doesn't matter if it's DP or HDMI.

I've been working with Dell support and found that the issue disappears when DP is changed from 1.2 to 1.1a, or HDMI is changed from 2.0 to 1.4. 

I don't believe it's a great solution as there are some restrictions rolling back to those versions, notably the fact that I'm now stuck at 30Hz, but it might point someone in the right direction with a better solution.

Jamie_ee,
This sounds like an issue with the quality of cable you're using. Despite being advertised for DP 1.4 or HDMI 2.0, the cable may be defective in that it works at the older spec but not the one advertised. This is fairly common with cheaper/older video cables.

we have the same problem with Dell WD19TB and WD22TB4

We use Dell Latitudes 5530 and 5330.

It's the same for us. We're using WD19TB, WD19TBs, and WD22TB4 and we're having this problem on each of them for a handful of the units we've issued out. The issue is alleviated when the dock is replaced with the regular WD19 or WD19s dock. The issue appears to be limited to the Dell Thunderbolt docking stations.
We are also using Latitude 5530 units, but we've also experienced this on a Dell Precision 7550.

We've tried multiple cables on multiple machines, including the cables that are supplied with the monitors from Dell. I'd be surprised if they are shipping monitors with incompatible cables.