tdr:

Using TP-Link Archer C7 v2 as access point only for a secondary VLAN.

Accidentally installed OpenWRT 21.02.0-rc3.

The VLAN is successfully configured on the switch, but the interface to the network adapter is being deleted when the network is brought up.

The bridge is still created, and the WLAN interfaces are added to it, but they're isolated from the wired network.

Relevant part of /etc/network:
config interface 'av'
option proto 'none'
option defaultroute '0'
option peerdns '0'
option type 'bridge'
option device 'br-av'

config device
option type 'bridge'
option name 'br-av'
list ports 'eth1.13'

Relevant log entries:
Mon Jun 21 19:28:59 2021 daemon.notice netifd: Interface 'av' is now down
Mon Jun 21 19:28:59 2021 daemon.notice netifd: Interface 'av' is disabled
Mon Jun 21 19:28:59 2021 daemon.notice netifd: Interface 'av' has link connectivity loss
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.733595] br-av: port 1(wlan0) entered blocking state
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.739187] br-av: port 1(wlan0) entered disabled state
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.744995] device wlan0 entered promiscuous mode
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.798697] br-av: port 2(wlan1) entered blocking state
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.804282] br-av: port 2(wlan1) entered disabled state
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.809938] device wlan1 entered promiscuous mode
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.843455] br-av: port 2(wlan1) entered blocking state
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.848828] br-av: port 2(wlan1) entered forwarding state
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.854421] br-av: port 1(wlan0) entered blocking state
Mon Jun 21 19:28:59 2021 kern.info kernel: [ 685.859745] br-av: port 1(wlan0) entered forwarding state
Mon Jun 21 19:28:59 2021 daemon.notice netifd: Interface 'av' is enabled
Mon Jun 21 19:28:59 2021 daemon.notice netifd: bridge 'br-av' link is up
Mon Jun 21 19:28:59 2021 daemon.notice netifd: Interface 'av' has link connectivity
Mon Jun 21 19:28:59 2021 daemon.notice netifd: Interface 'av' is setting up now
Mon Jun 21 19:28:59 2021 daemon.notice netifd: Interface 'av' is now up
Mon Jun 21 19:28:59 2021 user.notice firewall: Reloading firewall due to ifup of av (br-av)
Mon Jun 21 19:29:00 2021 kern.info kernel: [ 687.245150] br-av: port 3(eth1.13) entered blocking state
Mon Jun 21 19:29:00 2021 kern.info kernel: [ 687.250872] br-av: port 3(eth1.13) entered disabled state
Mon Jun 21 19:29:00 2021 kern.info kernel: [ 687.256843] device eth1.13 entered promiscuous mode
Mon Jun 21 19:29:00 2021 kern.info kernel: [ 687.262261] br-av: port 3(eth1.13) entered blocking state
Mon Jun 21 19:29:00 2021 kern.info kernel: [ 687.267783] br-av: port 3(eth1.13) entered forwarding state
Mon Jun 21 19:29:00 2021 kern.info kernel: [ 687.317661] device eth1.13 left promiscuous mode
Mon Jun 21 19:29:00 2021 kern.info kernel: [ 687.322553] br-av: port 3(eth1.13) entered disabled state

brctl show

bridge name bridge id STP enabled interfaces
br-av 7fff.f4f26daf0f44 no wlan0
wlan1
br-lan 7fff.f4f26daf0f45 no eth1.1

When the interface is set to the VLAN directly, then the physical interface doesn't get deleted, but the bridge is not created.