red rectangle under one superscript

Mon Feb 10 08:53:49 UTC 2020

I get a red rectangle under one superscript (see the attached lyx file)
I use debian bullseye and LyX Version 2.3.4 (Sonntag, 12. Januar 2020)

Perhaps a bit academic, since the pdf output seems to be ok. But still: 
Why the red mark?

Wolfgang
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