Steps to reproduce:
Go into spectator mode, spectate from the POV of a mob, go into creative mode. You'll go into creative mode but remain inside the mob, and you can place blocks.

This might be a deliberate feature but the UI does not reappear, so I think not.

In addition to this, the shader (when applicable, for instance endermen) remains set, but not in use. So when you leave the mob, the debug screen shows that you're still using the applicable shader.