Hello,
As as an update, I expiremented with changing the source for the rule to pass traffic from DMZMGMT to LAN, and instead of DMZMGMT subnet or the single IP of the machine behind the interface, I decided to use LAN subnet, which worked, since the machine behind DMZMGMT had an IP address in the same subnet.
So, if for some reason (again, maybe briding WLAN?) one wanted to put some firewall rules in place between two segments of the same subnet, this would work. I did not leave it this way, however.
I set it up to only pass ICMP and DNS from the machine in DMZMGMT to LAN, to reject all (other) traffic from DMZMGMT to LAN subnet. So in practice, the machine can still reach the internet and download upgrades, etc. It is also still reachable from the LAN subnet. But, all other traffic to it should be blocked by default.
stephenw10, thanks for the link to the patch. I actually had read that thread when researching the possible issues with bridging the vlans, but had not seen the logs that showed the link state going up and down and the attempts at re-assigning the IP address, etc. I will give the patch a try, and hopefully it will help with the stability.
Thanks again for all of your help and input! I really appreciate it.