nb. may have been a bad issue confirmation, as "Unless they've changed it since I last looked, you can't edit conflict with yourself." [1]

[1] https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&diff=1076115579&oldid=1076115051&diffmode=source

I've reproduced this misbehaviour in a sandbox, which may be invalid as both edits were by the same editor. I've also seen it in the wild with different editors. It's intermittent, but here are two scenarios which may make it more likely.
1: User:A starts editing page X; user:B moves page X to a new title Y, leaving a redirect X→Y; user:A publishes the change. This overwrites redirect X with a copy-paste move of a modified copy of Y.
2: User:A starts editing page X; user:B makes a quick section edit to X#Foo; user:A publishes the whole-page change. User:B's section changes are lost.

Edit conflicts are not supposed to trigger if the only conflict would be with yourself. This is a long-standing (mis)feature, and there are some proposals to change it, but it is the expected behavior for now. See discussion on T175745.

@Certes If both edits are by the same user, then that's the same as above. If you have an example of the bug occurring with two different users, or if you can reproduce it, then please file a task. I've searched for existing reports, and apparently there is a bug where a conflict is not detected when moving without leaving a redirect, which is what you might have seen in the wild: T25044.