Wireguard UI unreachable when used with network_mode: service:wireguard

[UltraBlackLinux]

Hey there,
I cannot for the life of me get the network_mode: service:wireguard statement to work. Whenever I insert that, the ports setting cannot be added to the wireguard-ui container, so you'd expect it to be accessible from within the wireguard container, but wrong. The request simply times out, or maybe you get a broken response (curl exits immediately, firefox loads for a long time), even with the correct port setting added to the wireguard container instead.
Any idea what's going on here? The config is basically stock, but this just does not want to work. I'm confused