If trying 2020.07 isn't too much trouble then we'd know if it's a regression or something that's always been there. But maybe it'd be enough to somehow draw @ezdiy's attention.
It's possible we already have a related open issue, see
https://github.com/koreader/koreader/issues/4747 and probably outdated
https://github.com/koreader/koreader/issues/3458.