It looks like this might be fixed in the next release:
https://bugs.launchpad.net/calibre/+bug/1921793
I look forward to checking that out in 5.15, which I think comes out at the end of this week.
Ironically, I had just ordered a Kobo ereader to test more thoroughly. Oh well, I know it's a good ereader, and good to have on on hand. :-)