The server is luckysb.org, forcing to protocol version 1.12.2 does not work, F3 shows server version 1.16.2.

this

I have identified the cause of this issue, and will be fixed in the next version, which I will release shortly. It was caused by a recent change made to ensure compatibility with ViaFabric's version detection.

Still having the exact same issue using multiconnect 1.3.20.
However, switching to an older version (i tried 1.16.1, multiconnect 1.3.8) and then back to 1.16.3 temporarily fixes the bug.
Maybe has something todo with caching?