There are multiple issues here.
1. Open attached score: Bracket_example_Concert_band_template.mscz
2. Zoom in and find the bracket at left of first page starting with Bb Trumpet parts.
3. Open Brackets palette and drag the one identified as "Square" to a staff somewhere in the middle of the bracket, like Horn 1 2.
Expected result: the square bracket will attach outside of the main bracket, looking like one of the many other examples visible. Actual result: the two brackets lie precisely on top of each other.
4. Drag another square bracket to Bb Trumpet 1, where the large bracket begins.
Expected result: the square bracket will attach outside of the main bracket. Actual result: the main bracket disappears!
5. Choose Edit -> Undo, or use corresponding shortcut.
Expected result: the square bracket will disappear again and the main bracket will return. Actual result: the square bracket disappears, but the main bracket has vanished beyond recovery. Undo command does not restore everything.
I should note that I'm not sure exactly how to reproduce these issues in a new score, but if you try it with the attached score this is what happens.