Don’t know of anyone else has found this but when spellcheck highlights a word as incorrectly spelt and you ask it to “learn spelling” it learns it until you close down. When you open up Scrivener at a later date the word has been unlearnt. I think that the same happens when you ask Scrivener to “ignore spelling”.
Also Scrivener does not recognise symbols. For example in my text I wrote the word “cliché”. Unless I am wrong - and I could be - Scrivener has no capacity for typing symbols and so I copy and pasted the “é” from Word. Scrivener did not recognise this.