In creating the program I found that it is a geometry issue.
My progress bar has a height of 7, which was fine with jdk 1.8, but in OpenJDK 11 nothing shows.
When I increase the height to 8, a get a very faint line. At its default size it's ok.
There is always a border around it (with both versions I believe) even when the border is set to 0 and transparent. Has the border thickness changed?
Not sure why the geometry has changed. I'm going to have to change my layouts to cope. A nuisance, but doable.