The 'Boss Bar' of champions renders multiple times per frame causing visual issues as seen here. This is due to wrong usage of RenderGameOverlayEvent in ClientEventHandler, which is likely by mistake. The event itself uses RenderGameOverlayEvent.BossInfo.Pre, but Pre is not actually a subclass of RenderGameOverlayEvent.BossInfo, but instead a subclass of RenderGameOverlayEvent itslef (to be honest, i don't even know why the compiler doesn't throw an error here). Therefore the event gets triggered by every RenderGameOverlayEvent in the Pre-Phase, resulting in multiple draw calls for your boss bar. For now, i see two possible solutions to this problem:
1)
Change RenderGameOverlayEvent.BossInfo.Pre to RenderGameOverlayEvent.PreLayer and check forForgeIngameGui.BOSS_HEALTH_ELEMENT == event.getOverlay(), else just return. To then prevent other boss bars from redering, you can create a new event, listening for RenderGameOverlayEvent.BossInfo, which is called for each boss bar individually, and cancel that event.
2)
You render your custom boss bar using the RenderGameOverlayEvent.BossInfo event, and only render it when the player is looking at the champions mob. Though, in order for this to work, you would need to register your boss bar somehow, and i don't know how that exactly works. But it would have the advantage, that other boss bars could be visible at the same time.

Huh, interesting. I guess no one ever realized because this is normally never noticeable. This seems like a simple enough fix, will probably have something out later today.