Velocity either throws "Incompatible client" or is stuck forever "Joining world" when using Neoforge or Forge respectively

[mannotcool]

Expected Behavior

I expect Velocity to connect me to the backend server.

Actual Behavior

If on Neoforge version 1.20.4 or 1.21.1 (I tested both), you will get an Incompatible client error despite being on the correct client.

Neoforge (1.20.4):

If on Forge, you will be indefinitely waiting on "Joining world" and the server will not recognize that you have tried to join. It will only get a disconnect packet ping when you click cancel in Minecraft.

Forge (1.21.1):

it just stays like this until you click cancel. And once you click cancel, then comes that disconnect server log

I should mention that I had Proxy Compatible Forge installed on the modded servers for all of this, so everything was set up fine.

Steps to Reproduce

1. Setup a Neoforge or Forge server running Minecraft version 1.20.4 or 1.21.1
2. Setup Velocity to connect to those servers with modern forwarding
3. Ensure that the modded server has Proxy Compatible Forge installed and modern forwarding is properly setup
4. Observe as you either get a version error while being on the same version or are stuck indefinitely loading.

Plugin List

Velocity:
No plugins

Backend Neoforge or Forge server:
Proxy Compatible Forge 1.1.7 (PCF)

Velocity Version

[05:59:32 INFO]: Velocity 3.4.0-SNAPSHOT (git-f986eb51-b472)
[05:59:32 INFO]: Copyright 2018-2023 Velocity Contributors. Velocity is licensed under the terms of the GNU General Public License v3.
[05:59:32 INFO]: PaperMC - GitHub

Additional Information

This is not a Forge or Neoforge issue. I downgraded my Velocity to build 470, and it worked without issue.

Velocity build 472 and build 471 give me issues. I have also tested with and without ambassador installed on Velocity, but that doesn't make any difference and shouldn't either.

[SeemWind]

same to me

[LevyWilson]

We were having the same issue. We had to use an older version of Velocity. All our servers are set to legacy forwarding, so it isn't just an issue with Modern forwarding.

[sys-32Dev]

+1 on this issue.

[tomalbrc]

Same issue with some fabric mods on 1.21.4