When I use libreoffice calc the squiggly red line won’t go away after I do the spell check corrections, but if i save it and reopen the file they disappear. Happens on all files that I have tested with a spelling error.
Version: 6.3.3.2 (x64)
Build ID: a64200df03143b798afd1ec74a12ab50359878ed
And what is your question (This sounds a bit like your intention is to inform developers about a bug or misbehavior)?
On version 6.3.4.2.0+, I get a similar problem, but it goes away if I scroll up or down. Sometimes, it clears after ‘one scroll’ in either direction, and sometimes it goes away until I scroll it ‘off screen’. Regardless, I never had to save and reopen the file.
I thought it might be related to my GPU or iGPU, but the problem is repeatable on both.
Try updating and see if it persists. If it does, but it’s like my situation, then such is life. 
Maybe a donation would help?