See e.g.
https://musicbrainz.org/work/056c2add-7a68-473f-8651-7f37e79b10e0/edits
after Edit #51147811

Context:
The main work had ordered parts A B1 B2 ... B7, but it should have two parts A and B, B having itself 7 parts B1 ... B7

• I created part B
• I edited B1 ... B7 to make them parts of B
  So now B has 7 parts B1 ... B7 ordered with numbers 2 to 8 (the part order was kept when I replaced the parent of B1 ... B7)

Two interface bugs, probably related:

• if I unselect "These relationships have a specific ordering", B1 ... B4 disappear from the list and only B5, B6, B7 appear
  If I reselect the ordering button: B5 disappears, B7 appears first with number "7." and then B6 with number "5."
  If I re-unselect, only B7 is left

• from the same initial state, if I click the up arrow on "2. B1" nothing happens. If I click on the down arrow B1 is properly moved to second position in the list with number 2. (and all other parts get the right number). If I click the up arrow on "2. B1" again, B1 disappears from the list

Debian/FF 58.0.1, all userscripts disabled