Steps to replicate the issue (include links if applicable):

• Perform a large number of edits at the same time (identical timestamp, perhaps through scripting)
• attempt to observe edits on your Special:Contributions page

What happens?:
Not all edits will be displayed in the list of contribs if they span across pages.

What should have happened instead?:
All contribs should be listed in Special:Contribs

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):
This was discovered by User:Fayenatic london, after using my MassRollback.js script on enwiki. This script performs a large number of rollbacks at once; in this case, they used it to roll back over 130 edits at 00:45 July 20, 2023.

Viewing Special:Contributions/Fayenatic london for this time period at a page size smaller than 500 will illustrate the issue (e.g. https://en.wikipedia.org/w/index.php?title=Special:Contributions/Fayenatic_london&target=Fayenatic+london&offset=20230720122220&limit=20 ).
At a limit of 20 contribs, we see: 13 edits at 00:45 on the first page, a full 20 contribs on the next page, and 15 contribs on the next, for a total of only 48 edits made at 00:45.
At a limit of 50 with the same initial offset, we see: 40 edits on the first page and 15 edits on the next page, for 65 total edits made at 00:45.
At a limit of 100 with the same initial offset, we see: 90 edits on the first page and 15 edits on the next page, for a total of 105 edits at 00:45.
At a limit of 500 with the same initial offset, we see: 136 edits on the first page at 00:45.

Obviously, these should all give consistent counts, but they don't. Fayenatic london reports that they would expect 139 total reverts, so it's possible that even on the 500 page, some edits are missing.