Describe the bug, provide any errors
https://paste.gg/p/anonymous/d7ad15dc011f43e6898ec37881664ee8

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

1. Start Paper-1.12.2 server with any BossBar plugin and BossBar function enabled (TAB's BossBar function / AirBar [premium plugin])
2. Start 1.8.9 minecraft client
3. Try to connect to the server
4. You will be kicked

Expected behaviour
Player should be able to join like on any version.

ViaVersion Dump
https://dump.viaversion.com/e5d95c26a86b078fefb65a9dfec4b378f6109782f96b318d8b864dd4e5e44faf

Additional server info
I am using Paper-1.12.2 for subserver, and FlameCord (one of BungeeCord's forks)

Checklist

• [ yes ] I have included a Viaversion dump
• [ yes ] If applicable, I have included a paste (not a screenshot) of the error
• [ yes ] I have tried a build from https://ci.viaversion.com/ and the issue still persists

This is an error in an incoming packet. I doubt it has anything to do with the boss bar plugin.
Is your Proxy up to date?
Are you running ViaVersion only on your Spigot server (not on the proxy)?
Are you sure this is not caused by a malicious client? Weird client settings?

Actually this duplicates #240

I am sure it happens because of BossBar function
I tried using AirBar, tried joining - got kicked out
Disabled ONLY AirBar plugin, tried joining multiple times - everything worked fine

Tried removing AirBar and enabled TAB's BossBar function instead, tried joining - got kicked out
Disabled TAB's BossBar function, tried joining multiple times - everything worked fine

Actually this duplicates #240

That's not AT ALL similar to that issue.