Palettes missing except for a single solid line passing through the whole section in version 3.4.2
See image attached. Resetting to factory settings, restarting, and reinstalling didn't help. Theoretically, that means that just installing the program should be a reliable way of reproducing this issue, but I imagine that isn't the case since a bug like this would be immediately reported.
OS: Arch Linux, Arch.: x86_64, MuseScore version (64-bit): 3.4.2., revision: 148e43f