When appending mediums to a target release, the "Merge releases" page appears to reorder the the listed mediums, and alter the user-assigned medium numbers after a failed submission.

Steps to reproduce

1. Log in to MusicBrainz.
2. Add 2 or more releases for merging so that you arrive at the "Merge releases" page.
3. Select a target release.
4. Optional: Change the value of "New position" and/or "New disc title" for one or more mediums.
5. Click "Enter edit" with no edit note so that the submission fails.
6. When the page updates with the error message, see that the sequence of the mediums has now changed, and any modified values for "New position" and "New disc title" have shifted to another medium.
   1. If nothing has changed, click "Enter edit" once more. See that the list has changed.
7. Correct the value of "New position" for each medium.
8. Enter a valid edit note, then click "Enter edit" so that the submission succeeds.
9. See that the submitted changes do not reflect the values entered in step 7.