Spell check as you go (f6) is a little flakey.

Perry Kibler 8 jaar geleden bijgewerkt door Keith David Bershatsky 7 jaar geleden 3
If you're working with spell-check-as-you-go on (f6), sometimes flagged words will go dark (which in my theme, means they are invisible), other times, they are white text (as they should be) with red underlines. If they do go dark, simply continuing to type will fix the problem, but it's a shame to have such a beautiful product marred by such a silly bug.

I am seeing this behavior too. I'm using the Soda Dark theme along with the Tomorrow Night Bright colors and having the misspelled words disappear and reappear as I type or move around the file I'm editing is annoying. 

There is now a viable workaround -- a spell check alternative.