see also https://bugs.kde.org/show_bug.cgi?id=373232
Horizontal lines with fractional HiDPI scaling in konsole.
This bug still in FreeBSD 12.2.
This should go away with Qt5 5.15.1 or 5.15.2 either of which should land hopefully before the end of the year in the ports tree.
I have 5.15.1 running locally, and have not had the issue anymore.