I have the Note 10 and I have the same issue!! I can't use my business page and I keep getting a bubble at the bottom of my screen that says TEST NT INSTANT when I'm on the business page. I googled it and found your question. I really wish someone had more info on this! It's driving me crazy.
I'm getting the same "test nt instant" in a grey oval at the bottom of my Facebook business page. It fades after a few seconds. I haven't found any mention anywhere other than your post.
- Where is Android 11