Some edit conflicts shows a diff against the old text from before the edit conflicts, which results in a diff, which shows only the own added lines.
When saving this edit, the conflict edit is lost.
This sounds like a lag issue. The editpage does not load the text from master to get the very current version to show here.
Please have a look here. Thanks.
Version: 1.21.x
Severity: major
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=10573
https://bugzilla.wikimedia.org/show_bug.cgi?id=56849