AP adoption failed over VLAN Controller in Docker

Hi, I'm trying to understand the following issue:

My setup: Router running OpenWRT with the Unifi Controller running in a docker container, a switch and a UAP-AC-LR AP.

The router has a VLAN device with vlanid 20, static IP 192.168.20.1/24 and DHCP server. The switch ports for router and AP are configured as trunk ports (the AP one with PVID 20). Devices can get an IP and communicate with the router. The AP shows up in the dashboard, gets an IP in the subnet, but adoption always fails.

I can also not ssh into the AP. It's on factory settings. Is this expected?

I've configured the inform IP in the controller to 192.168.20.1, so reachable for the AP. I've also created a network in the controller with the VLAN id 20 and set that as the Management VLAN (not sure if this is necessary).

Today I found out, that when I assign an IP address to the physical parent interface of the VLAN interface of my router, the AP gets adopted.

I don't understand what that would change and why it would suddenly work. It's also not something I planned on doing. I'm not using the default VLAN anywhere else.