Unable to connect to 1.20.1 server with 1.21 client (`update/1.21` branch)

[helpimnotdrowning]

/viaversion dump Output

The command did not produce any output.

Console Error

https://mclo.gs/V9DL72U

Bug Description

I'm unable to connect to my 1.20.1 Purpur server with a locally-built clone of update/1.21, using only the jar that appears in build/libs (builds as ViaFabric-0.4.14+null-1.21.jar).

Steps to Reproduce

1. Build new JAR of branch update/1.21 (not using latest Actions build, has broken command preventing startup)
2. Include newly built build/libs/ViaFabric-0.4.14+null-1.21.jar into mods/ directory
3. Start game, version can be set to either "AUTO" or "1.20.1"
4. Try to connect to (Purpur?) 1.20.1 server
5. Observe either in-game message saying "Outdated server! I'm still on 1.20.1" or (less likely) error "java.net.SocketException: Connection reset" (see mclo.gs log)

Expected Behavior

Successful (more or less, since the branch may/may not be ready) connection to server

Additional Server Info

Client:
Minecraft	1.21
Fabric Loader	0.15.11
Fabric API	0.100.1+1.21
ViaFabric	0.4.14+null-1.21, client-side mode both enabled and disabled
(includes) ViaVerision	viaversion-5.0.0-SNAPSHOT-downgraded-1.8-shaded-1.8.jar

Server:
Minecraft	1.20.1
Purpur	git-Purpur-2062 (MC: 1.20.1)

Checklist

• [ ] I have checked if this is specific to ViaFabric platform, and doesn't happen when running it on other platforms (Paper/Sponge/Velocity/Waterfall).
• [X] I have included a ViaVersion dump.
• [X] If applicable, I have included a paste (not a screenshot) of the error.
• [X] I have tried the latest build(s) from https://ci.viaversion.com/ and the issue still persists.

[FlorianMichael]

You can't use this branch without preview ViaVersion/ViaBackwards