That doesn’t sound like a (trying to do something) feature I’ve ever heard of.
Quick question : do you have pro-writing-aid or something like that installed?
(I’m on Windows, I can’t really help, other than helping others help you with my question. Which question came to me as so obvious that I had to ask.)
Unfortunately, I’m too new here to be allowed to post images, but it’s the fourth item in the ProWritingAid settings (accessible from the menu bar): Show Context Thesaurus.
It’s odd that it only seemed to pop up in Scrivener, and not Word, TextEdit, nor Notes, other apps I use for writing.
The setting option to uncheck. Click the ProWritingAid icon in the menu (near the time/date in top right on Mac or bottom right on Windows) and select “Settings” to get here.