I posted this at Viaversion but they seem to say it's a Fabric api-bug: ViaVersion/ViaVersion#4575

Bug Description
I have a Velocity-network. Setup: 1.21.5 Lobby-server running paper and a 1.21.7 Fabric server. When landing from enough blocks high you get disconnected with the following errors:

In server console:
Missing 1.21.6 blockstate for 1.21.7 blockstate -1

On the client:
[Netty Client IO https://github.com/ViaVersion/ViaVersion/issues/2/INFO]: Stopping tunnel to 127.0.0.1/127.0.0.1:58270
[Render thread/WARN]: Client disconnected with reason: Internal Exception: io.netty.handler.codec.DecoderException: java.io.IOException: Packet play/clientbound/minecraft:level_particles (class_2675) was larger than I expected, found 9 bytes extra whilst reading packet clientbound/minecraft:level_particles

I connect with a 1.21.7 Fabric client and no errors are shows in the 1.21.5 lobby server except for MBAC lost connection: Disconnected. MBAC left the game.

I even tried to make a clean Fabric client with only fabric.api, viafabric and via version and a clean Fabric server with the same mods.

Steps to Reproduce
Make a velocity network.
Lobby-server on paper 1.21.5
Game-server on fabric 1.21.7
Connect with 1.21.7-client
Go up 6-7 blocks in the air.
Fall down.

Expected Behavior
Sound of touching the ground. maybe take damage (tried both in survival and creative mode with the same outcome).

While this is an issue, it is neither a bug in fabric API, nor a bug in via version, but rather an incompatibility between via version and fabric API. There has been discussion (thus far inconclusive) on what to do about this, but currently you can uninstall either via version, or fabric API to fix this. You can also just uninstall the module causing the incompatibility with via version.
Keep in mind that fabric API doesn't officially have any sort of cross version guarantees, nor does via version have any guarantees with fapi. I'll leave this for someone else to close once a conclusion is reached.

Also, I'm not sure what source of information the via version people have, but as far as I know, we haven't made any promises about fixing this, only looking into the implications of it (might be wrong lol) :)

Also, I'm not sure what source of information the via version people have, but as far as I know, we haven't made any promises about fixing this, only looking into the implications of it (might be wrong lol) :)

this sounds like a "wontfix" issue to me even though this has managed to break viaversion on fabric since 1.21.2+. (initially due to networking related bugs but this was partially solved)