Does this occur during a replay or when you start up the game? Should probably add a bit more detail so the developers know what's causing it.

Also, ViaVersion on your Minecraft client as a Forge mod? How did you pull that off?

It happens because SpongeForge is loading ViaVersion from ReplayMod jar

Aight, I don't work with Sponge so I guess I'll let the developers take over this one.