I remember managing to get this issue solved on my computer. Looks like a QT issue. I may not remember exactly how I fixed it, but try the solutions in this thread and see if it works: https://lmms.io/forum/viewtopic.php?t=34104
In particular run export QT_AUTO_SCREEN_SCALE_FACTOR=0
and then run LMMS from the same terminal.
Also in response to other comments, LMMS is actively being worked on (though updates have been slow) -- the last commit on the master branch was from 3 days ago, so that suggests that the project isn't abandoned but development's just rolling slowly.
Hey! Seems my instance can receive posts from yours; both !godot@programming.dev and !programming@programming.dev seem to show the latest posts (newest one is from 19 hours ago). Can this federate back?
edit: I made a whole comment on the issue page without realizing that 19 hours ago was also when you fixed federation... congrats on getting things to work! And maybe my coffee shouldn't be as strong...