RSTP switches reporting they're in different regions

In a network I've had to inventory recently I found an interesting fact which I couldn't explain. There were 2 switches, both configured for RSTP, yet both were convinced they were in different regions.

The switches are a HPE 5710 48XGT 6QS+/2QS28 and a Dell N1108T-ON. There weren't any other STP aware switches at the moment of inventory. The HPE is a stack and supposed to be the core of this network.

Due to some oversight of the person installing the devices, STP priorities were default and due to its lower MAC-address, the Dell became root. The HPE reported the Dell as root, but it also stated it was a regional root itself.

After lowering the priority of the HPE to 4096 it became root, the Dell agreed on the HPE being root, but also the Dell sees itself as a regional root.

In the new situation I also did some capturing. Wireshark does not show any regional information in the BPDUs. So why do these switches keep reporting they are in different regions?

Following are some show and display outputs of the new situation, the old situation is no longer possible to recreate as this is a live environment.

Basic RSTP info for HPE

display stp
-------[CIST Global Info][Mode RSTP]-------
 Bridge ID           : 4096.dc68-0c77-e58d
 Bridge times        : Hello 2s MaxAge 20s FwdDelay 15s MaxHops 20
 Root ID/ERPC        : 4096.dc68-0c77-e58d, 0
 RegRoot ID/IRPC     : 4096.dc68-0c77-e58d, 0
 RootPort ID         : 0.0
 BPDU-Protection     : Disabled
 Bridge Config-
 Digest-Snooping     : Disabled
 TC or TCN received  : 589
 Time since last TC  : 6 days 2h:34m:1s

Port towards the Dell (notice the Boundary)

----[Port55(Ten-GigabitEthernet1/0/50:1)][FORWARDING]----
 Port protocol       : Enabled
 Port role           : Designated Port (Boundary)
 Port ID             : 128.55
 Port cost(Dot1T)    : Config=auto, Active=20000
 Desg.bridge/port    : 4096.dc68-0c77-e58d, 128.55
 Port edged          : Config=disabled, Active=disabled
 Point-to-Point      : Config=auto, Active=true
 Transmit limit      : 10 packets/hello-time
 TC-Restriction      : Disabled
 Role-Restriction    : Disabled
 Protection type     : Config=none, Active=none
 MST BPDU format     : Config=auto, Active=802.1s
 Port Config-
 Digest-Snooping     : Disabled
 Rapid transition    : False
 Num of VLANs mapped : 1
 Port times          : Hello 2s MaxAge 20s FwdDelay 15s MsgAge 0s RemHops 20
 BPDU sent           : 268774
          TCN: 0, Config: 0, RST: 268774, MST: 0
 BPDU received       : 12598679
          TCN: 0, Config: 0, RST: 12598679, MST: 0

Basic RSTP info for Dell (notice the regional root)

show spanning-tree

Spanning Tree: Enabled - BPDU Flooding: Disabled - Portfast BPDU Filtering: Disabled
Mode: rstp
CST Regional Root:        80:00:68:4F:64:1F:70:81
Regional Root Path Cost:  0
ROOT ID
              Priority        4096
              Address         DC68.0C77.E58D
              Path Cost       20000
              Root Port       Gi1/0/11
              Hello Time: 2s Max Age: 20s Forward Delay: 15s Transmit Hold Count: 6s
              Bridge Max Hops: 20
Bridge ID
              Priority        32768
              Address         684F.641F.7081
              Hello Time: 2s Max Age: 20s Forward Delay: 15s Transmit Hold Count: 6s

Port towards the HPE (Dell shows the designated port ID from the HPE, but the designated bridge priority and address are its own)

Port: Gi1/0/11 Enabled
State: Forwarding                                Role: Root
Port ID: 128.11                                  Port Cost: 20000
Root Protection: No
Designated Bridge Priority: 32768                Address: DC68.0C77.E58D
Designated Port ID: 128.55                       Designated Path Cost: 0
CST Regional Root: 10:00:DC:68:0C:77:E5:8D       CST Port Cost: 0
BPDUs: Sent: 12598695, Received: 269035