"Expandable" and "expanded" AT-SPI2 (Linux) states missing from state set of palettes
If you look in the attached screen shot of using the Accerciser accessibility inspector to examine MuseScore, you'll notice that expanded palettes (here "Articulations") lack both the "expandable" and the "expanded" states.
Orca checks things like list items and tree items for the "expandable" state when these items initially gain focus. If the "expandable" state is found, Orca then looks for the "expanded" state. If it's found, Orca includes "expanded" after the accessible name; otherwise, it presents "collapsed." But if the "expandable" state itself is missing, neither presentation occurs.
In addition, when the already-focused object changes states, Orca expects (and usually gets) an object:state-changed:expanded event (The event's detail1 field being a boolean int to indicate if it's now expanded.) When this occurs, Orca says "expanded" for detail1 = 1 and "collapsed" for detail1 = 0. These events are not being emitted.
It is possible that adding the accessible states to the state set and then updating them will automatically cause Qt to send the expected state-changed event. Though I couldn't tell you for certain.
Please also remove the "expanded" (or "collapsed" or ...) from the accessible name. When the missing states and events are fixed, having the state in the name will just result in double presentation by screen readers.