Steps to reproduce:
- Make sure to run into a conflict.
- Do an edit on the left, yellow side. It does not matter if you confirm the edit with the checkbox button or not.
- Click "Show changes". The edit might show up in the diff on top, but it is lost in the conflict resolution interface below.
- Whatever you do now, the edit is lost.
As reported by @Vort here at https://www.mediawiki.org/wiki/Topic:Up5ypjzsdwtu9s35.
 The user report is slightly different here.