Yeah. A big thank you to @dekani for the bug report and continuous feedback. And of course to @NiLuJe for his awesome job, including a new - modern - toolchain for PB users instead of the old obrey one.
The bug, described in
https://github.com/koreader/koreader/issues/5861, is still affecting 2020.02 release but new nightlies work as intended.