Local discovery was partly fixed in v1.16.0.5. The problem was Android 11 blocking apps from receiving multicast packets from the network. The app now holds the permission to create WifiManager.MulticastLock which allows receiving multicast packets from the local network thus lets the affected Android 11 discover other devices running Syncthing’s local discovery again.
Please note that Android 11 devices cannot discover other Android 11 devices using Syncthing’s local discovery mechanism because sending out multicast packets is still blocked by the OS. I didn’t figure out a way yet to let our NDK executable perform the sending of multicast packets.
Beside Android Developer documentation, here’s also a good writeup about multicast on Android in general: UDP Multicast on Android - CodeIsland (not Android 11 specific)