Line Spacing over 100% and under 50% breaks text rendering

On Ubuntu 20.04 as well as on Windows 10, Qt Creator 9.0.0-rc1 (8.0.84) with line spacing larger than 100% disables the line wrapping completely. 

With 120% as well as 101%:

Values from 100% down to ca. 70% seem to work:

Whereas 50% yields completely the opposite:

I repeated the test with the standard font "Source Code Pro" which came with Ubuntu, and it has the same problems, so apparently it's not the font file. 

According to apt, I have libfreetype6 2.10.1, if that's relevant.

The same problem is reproducible on Windows 10 with JetBrains Mono.