See: ViaVersion/ViaBackwards#219

How can we reproduce it?
Steps to reproduce the behavior:

1. Login on 1.16.1'

2. Click on space

3. Fly

Screenshots

client: 1.15.2
mods:

Server: vanila 1.16.1

Seems to be ViaBackwards fault (sending invalid minecraft:climbable tags)

With ProtocolSupport there is the same problem!

And only if I have Fabric API in my mod folder

1.15 Fabric API reimplements the minecraft:climbable tag. These protocol proxies should be sending the right tags to the right clients, that's the issue here. Too many of them don't, this isn't the first issue it's caused.