Starting a Mercenary fight removes banner buffs

All banner buffs are removed at the start of a Mercenary fight. This is a bug.

Here is video evidence of this bug.

https://youtu.be/REuKVmzLVU0

Here is an example of how starting a mercenary fight removes all banner buffs. This seems to happen every mercenary fight. I use banners as a major defensive layer and it has resulted in many deaths when starting a mercenary fight. Something about the start removes all banner buffs. I'm curious if it removes other buffs too. Could be the reason these guys keep one-shotting players who are normally very tanky.