This video shows it best:
Happens on xbox one x(my console) too. Cant progress because of it.
If you dont feel like watching the video, for whatever reason, this is the katagawa jr boss fight. Where zer0 is being accused of being a traitor.
When katagawa jr reaches approximately 20 to 25% health left he goes invisible and stops moving. You cant kill him. He continuously regains health and shield. Whilst remaining in one spot on the map.