I just tried this with a fresh installation of Linux Mint 19.1 in a virtual machine and managed to reproduce the bug.
It seems like openjfx cannot be loaded if the default java version is java 10 (or java 11).
- When only openjdk-8-jre was installed it perfectly worked
- When openjdk-11-jre (default-jre) was installed and set as default, it did not work, even when launching it with openjdk-8-jre from the context menu
- When openjdk-11-jre was installed and openjdk-8-jre set as default, it works perfectly fine
(all this still with openjfx installed and default java versions set via update-java-alternatives)