I couldn't find anything about this on Google, but it's starting to drive me a bit mad. I'm using the Mac version of Scrivener, fully licensed etc.. and I'm finding that Scrivener will occasionally (every couple paragraphs) decide to put a red squiggly under part of a correctly spelt word. For example, I just had this happen to the word 'three' it underlined it like this: three, it then suggested the word three, which I chose and ended up with threee. The only way I can make Scrivener realise the word is actually spelt correctly is by putting a space at the end of the squiggly and then rejoining the word. I hate seeing red squiggles, and I like to fix them as quickly as I can, so having to fix these is really disruptive for me. Has anyone else encountered this issue?
I have not encountered this issue myself, so I cannot say for sure what's happening. But I have some comments. First, the Mac version of Scrivener does not have its own spell-checker. It's using the spell-checker of macOS. So maybe try another app to see if this problem is Scrivener specific or not. Nearly all text-/word-processing apps on the Mac use the same macOS subroutines for spell-checking. Second, for the given example, the macOS spell-checker must think of the underlined 'thre' as a single word, with an additional 'e' attached ... and I cannot think of a way how that happened. I mean, the rules for word separators in everyday language are relatively simple (space or a non-letter symbol)... Sorry if I cannot be of more help.
Could it be related to style or language shifts within a word causing spell check to assume an invisible break? This might be related - https://wordmvp.com/Mac/SpellCheck.html#NoProofing
I'm beginning to think it's a bug in Scrivener. I've started to notice that they will show up in the windowed editor, but not the full-screen editor and visa versa.