Two-chord tremolo removed when chords are assigned to a different voice, but not when exchanging voices
1) Follow handbook steps (https://musescore.org/en/handbook/3/tremolo) to create a two-chord tremolo in voice 1.
2) Select the two chords.
3) Assign to a different voice (e.g., press Ctrl-Alt-3 to assign to voice 3).
4) The voices are assigned to the new voice, but revert to half-duration and the tremolo is removed.
Note that the tremolo remains if you select the two chords and exchange with another voice via the tools menu.