I saw this because I had noticed that “learn Spelling” was not working in RC10. After I right-clicked and choose “learn spelling” the red dots/underline persisted (as the previous poster noted). When I checked Options | Corrections | Personal World List, I could see that the word had not been added to my personal dictionary (wordlists.ini)
Same thing happened when I launched a spelling check from the menu: the first instance of the word (a proper name, in my case) was flagged.-- I chose “learn spelling” and continued the check, but the next instance was still flagged, etc.
The problem was definitely with “wordlists.ini” not being used properly, because even adding the word manually didn’t work: Scrivener continued to flag it, and words I had added in earlier betas are now being flagged as misspelt.
However, the problem was that I had started the session with the default US English dictionary (as it’s a fresh, RC10) install, but I usually use the UK/GB English dictionary. I had downloaded and selected the UK option and continued the session when I noticed the problem described above. Restarting Scrivener fixed it.
So, if others see this problem, try restarting after installing and selecting your preferred dictionary. Hope that helps