I don't understand this bug that I've been complaining since WP 16.. I write in Catalan, the spellchecker flags correctly words that are in its list as incorrectly spelled! I click on correct but the squiggly line doesn't disappear. So I either need to add or skip the word.
Also, the spellchecker still doesn't recognize the L geminada (the L with a dot in the middle followed by another L) I realize I'm the only one writing that language in WP but at least you can incorporate the Language Tools (https://languagetool.org/) addon to correct that bug and to incorporate some grammar checking.