I have a 2018 MacBook Pro. For quite a while, when I'm using the message app it can start being glitchy - underlining correctly spelled words as mis-spelled, correcting words and then going back to the typo version when I'm further doing the message.. I experience that for years including when I updated macOS. It only ever occurred with the messages app, every other app the spell check worked normally.
I recently completely erased my computer (formatted the SSD) and updated to Monterey, set it up as new to try to clear all the bugs. I reinstalled all the apps I use manually, and recovered my data that way as well - so installing the apps fresh, and data back to my documents folder by dragging it in finder from a backup drive. I figured that by getting going again this way instead of a time machine recovery it should get my computer back to a 'new' bug free state... and it has generally been a lot snappier at least. BUT as of today the messages app spell check glitch is back. So that tells me that it likely has to do with one of the apps I've reinstalled, and there's not that much that runs in the background, so that;'s why I suspect Better Touch Tool. Has anyone else experienced this?