When I finally made sense of the VLANs and bridges, got some test interfaces working, I decided to start fresh but either I forgot what I had done, or something is wrong with my router because none of the interfaces will respond on their statically assigned addresses.
However, I got an emergency interface (y'know…because I'm a newbie) and using that I logged in to the router, checked around to find nothing unusual (I think) except for the lack of addresses that should've been configured. So, I issued an ip a add… command and that interface worked again. Same with the next one, and the next one, and so on.
There's also duplicates; they appears as soon as the interface gets its address, but I assume that's because of the misconfig.
This a is tiny small part of the config, the rest is a copy of these sections, I'm not adding it to keep it short.
# /etc/config/network
config interface 'Management'
option device 'eth1'
option proto 'static'
option ipaddr '10.9.0.251'
option netmask '255.255.255.0'
option broadcast '10.9.0.255'
list ip6addr 'xxxx:xxx:xxxx:xxx::xx/120'
option delegate '0'
option defaultroute '0'
option metric '200'
…
config device
option type 'bridge'
option name 'mainbridge'
list ports 'eth0'
option stp '1'
…
config bridge-vlan
option device 'mainbridge'
option vlan '9'
list ports 'eth0:t'
…
config interface 'z0900'
option proto 'static'
option device 'mainbridge.9'
option ipaddr '10.9.0.2'
option netmask '255.255.255.0'
option broadcast '10.9.0.255'
list ip6addr 'xxxx:xxx:xxxx:xxx::/120'
option ip6prefix 'xxxx:xxx:xxxx:xxx::/120'
While closing a ton of browser tabs, I believe I caught a glimpse of something that said order was important, I'm not sure, it was a private session I think because I couldn't find it again in the history.
Regardless, the only interface I could move, I did, and it didn't make any difference. "Management" is eth1, above of the main interface, eth0, a bridge-based route-on-a-stick-type interface.
It's either:
- eth0
-- eth0.1
-- eth0.2
-- eth0.…
- eth1
or:
- eth1
- eth0
-- eth0.1
-- eth0.2
-- eth0.…
Appreciate your help. 
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[SETUP] OpenWRT v22.03.0-rc6 is running on a vSphere VM hw. v15. All vNICs are VMXNET3. eth0 is a trunk with all VLANs, eth1 is only VLAN 9 for recovery/lockout access.
[OBSERVATIONS] Other routers that work similarly — i.e; bridge VLAN filtering e.g; Mikrotik CHR, VyOS — work fine, as well as others that don't rely on bridges such as pfSense/OPNsense. It doesn't seem to be a hardware problem.