Debian/Ubuntu have "libopenjfx" package install OpenJFX 11, which is incompatible with OpenJDK 8 and therefore with current releases of BlueJ/Greenfoot. They should instead provide separate OpenJFX-8 and OpenJFX-11 packages.
Upstream Ubuntu bug: https://bugs.launchpad.net/ubuntu/+source/openjfx/+bug/1799946
Upstream Debian bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=910407
Note the comment in Debian bug indicates they may not fix.