Incorrect Polymeter Handling
When applying different time signature to different staves, the barline still line up while it shouldn't (since the bar length of each staff is different), and it's also played back this way. Technically it would make the time signature being 4/3, 4/4 and 4/5 (rather than 3/4, 4/4 and 5/4) but when using polymeter it's also impossible to change the displayed value of the time signature so this feature is useless.
Also, when changing time signature the staff doesn't re-bar properly but that probably is already in another issue.
(I'm marking this "Major" since even though polymeter support is more of an incomplete feature, it doesn't work correctly regardless.)