font in LibreOffice messages

I think this question was answered some time ago, possibly in a different (debian) forum, but I can't find the answer and it's bugging me. Some messages give me alternate ways to see the message while others don't (e.g. some Base messages have a pulldown for a fuller message).

I'm using 5.1.3.2 on a Debian/Stretch AMD64 system. The text in a lot of the pop-up messages (e.g. error messages) is unreadable. It's like the font is too small for there to be enough pixels to render it properly or something.

Possibly its the font size needs adjusting or possibly the font itself is corrupt (although it only happens in LibreOffice so it's not a font the desktop uses).

Any ideas on how to track this down and fix it?

I may be way off track here, but some years (quite a few) ago I had similar. There was a desktop setting that set a pt. size below which fonts were not rendered (properly). I dropped this to 4.
Don't ask me where, I will look about but have KDE on openSuse.
Steve

Whatever it is, it's not obvious in the Plasma 5 desktop settings and only seems to affect LibreOffice.