Describe the bug, provide any errors
A clear and concise description of what the bug is. Can you https://paste.gg/ the error?

The bug is when i install Viaversion , ViabackWards and viarewind , and when i join the server with 1.8.8 client on my 1.12.2 server it kick me instantly for [Proxy] Lost connection to the server (screen : https://prnt.sc/vaofn6)

I use Travetive for proxy , i try to change it and use normal bungeecord , flamecord ect but same issue.
And server version i use Paper 1.12.2.

I got 0 plugins on my bungeecord.
I got Only ViaVersion , ViaRewind and ViaBackWards my spigot server.

How can we reproduce it?
To reproduce the player kick issue ,
1 - just setup a bungeecord
2 - setup a 1.12.2 paper server
3- install viaversion , viarewind and viabackwards on it (spigot side)
4 - Join with a <= 1.8.9 client

Expected behaviour
A clear and concise description of what you expected to happen.

I expect it to be fixed
Screenshots
If applicable, add screenshots to help explain your problem.*

With the kick issue on <= 1.8.9 client i got just 2 screens 1 minecraft side and on bungeecord side
Minecraft Side: https://prnt.sc/vaoixp
Bungeecord Side (Console) : https://prnt.sc/vaoj6l
No console errors proxy / spigot side.
ViaVersion Dump:

Type /viaversion dump, and put the link here.
https://dump.viaversion.com/dd77e09a3d415f490c96cf64f49d830ebc59c17a6d7a646b3553ce3e1feae461
Additional server info
Do you use a proxy (eg. BungeeCord)? What software do you use and what plugins?
i use Travetine lasted for this test , but alredy tryed to use FlameCord or a normal Bungeecord. i dont have plugins proxy side.

Checklist

I have included a ViaVersion dump

If applicable, I have included a paste (not a screenshot) of the error

I have tried a build from https://ci.viaversion.com/ and the issue still persists

I install lasted version of all the plugins

Good luck for fixing it. and Srry for my english i'm french

Bug can be replicated on our server as well. We're reverting to 1.8-1.15.2 support on a legacy version of VV for now until this gets fixed.

if that can help you the issue is caused by using 1.12.2 spigot bossbar api :
bossbar = new Bar();
bossbar.bar = Bukkit.createBossBar("", BarColor.RED, BarStyle.SOLID);

Confirm. I have in the course of the same because of the boss of the bar kicks on 1.8
Server version 1.12.2