Unwanted TAB string change not "undo"able

After an unwanted string change occurs as experienced via issue #303789: Changing notes illogically changes / resets string in tab, that change cannot be undone via "undo".
Steps to reproduce:

1. Open the score in the issue mentioned above and follow the two steps described there
2. Hit Undo

Actual result:
The score is not returned to its initial state, the tabbed note is still on the wrong string. Interestingly enough the * besides the file name tab disappears - though the score is not the same than it was before.

Expected result:
The score is returned to its initial state.

Undecided whether this is major or critical, but it think it's at least the former - as this can completely destroy user trust in the software: When editing a huge fingerstyle guitar score with complex movements - such as the one i linked here (those are only the first 20 seconds out of 4 minutes), finding that you (or the software to the the bug linked above) made a mistake and hit undo, you want to be sure it exactly restores the fret notations as you meticiously arranged them in the first place.