U4025QW problems daisy chaining using inputs from two PCs

Context

I have two Windows laptops connected to a Dell U4025QW monitor, via the in-built KVM.

Laptop 1 > connected to U4025QW via Port #6 (TB4 upstream)

Laptop 2 > connected to U4025QW via Port #4 (DisplayPort in) + USB (for use of peripherals)

I've then created a daisy chain from U4025QW (Monitor 1) to a Dell P2425DE (Monitor 2), using the following ports:

Monitor 1 (primary - U4025QW): Port #5 (TB4 downstream for daisy chaining)

>

Monitor 2 (secondary - P2425DE): Port #5 (DisplayPort in)

I've ensured that MST is turned on for Monitor 2 (secondary - P2425DE). My understanding is that MST is on, by default, when a source is plugged into Port #5 on Monitor 1 (primary - U4025QW).

Problem

From here, I am only able to view video on both monitors from Laptop 1 (connected to U4025QW via TB4).

From laptop 2 (connected to U4025QW via DisplayPort in), I am only able to view video on my primary monitor (U4025QW). Monitor 2 (P2425DE) simply displays a "No DP signal from your device" message on the screen.

I've tried literally every combination of cable / third-party hub / etc. but nothing seems to have resolved my issue. Hoping someone here can shed some light!

Question

What I'm wondering is: does the U4025QW's Port #5 (TB4 downstream) broadcast video from whatever the current active source is (e.g. TB4 upstream or DisplayPort in), or does it ONLY broadcast from the source connected via Port #6 (TB4 upstream). I'm unable to find anything that specifically addresses this question in the documentation.

If the former, what am I doing wrong!?

Thank you in advance!!