When the conflict includes something like ~~~~ (the users signature on talk pages), this is shown as is in the diff in the TwoColConflict interface. The pre-save transformations are not run before creating the diff.
https://www.mediawiki.org/wiki/Pre-save_transforms
Expected: The diff shows the users signature expanded.
Note: This sounds like it is the same issue we fixed before, but that was for the preview, which is JavaScript code. This bug here is in the backend.