I am experiencing this issue on a Pixel 9a with Android 16 (GrapheneOS). Over the last weeks I first git bisected the wrapper and then syncthing native. The commit causing this issue is 9d425b05880236928c561da0b7152e41d795a5ac in Syncthing Native. It uses a new library, anet to fix some methods regarding network interfaces and addresses for Android 10+.
Instead of downgrading to v1, I found this workaround which resolves the issue for me: Disabling Settings -> Syncthing Options -> Local Discovery.
I’d be interested to hear if this workaround works for other people here having this issue.
I am still trying to figure out if the root cause is in the commit that I found (or rather the anet library) or if the root cause is in another part of the Local Discovery feature and the fix for Local Discovery on Android 10+ only made the issue visible on newer Android versions.