This bug is back with 3.1.
It was solved in 3.0 with one of the updates.
Ooops. How could the fix escape from merging to the main branch?
Thanks for reporting, will be fixed soon.
The update in ZIP archive can be downloaded from here: https://storage.googleapis.com/dl.ganttproject.biz/updates/update-3101.zip
One can install it by unzipping in .ganttproject.d/updates/ folder in the operating system user home folder. It will be pushed to the update channel tonight.
The update is published now.