GitHub took it upon themselves to change email addresses and author names when merging via the UI buttons like "Squash and Merge" in 2018 and then again in 2019. See <https://github.com/isaacs/github/issues/1368> for the tedious details.

Essentially the post-2019 behaviour seems to be that where possible with "Squash and Merge" they will set noreply@github as the committer so that they can sign the merged commit themselves, and set author name & email to what they have recorded for the GH account involved (and the signature is then a record that GH have verified that account's involvement).

Personally I think it is shocking that they ignore the name and email address that the actual author of the commit has selected. This is both a violation of the author's intentions -- for example, you may set work and personal email addresses in different repositories as discussed here, but GitHub will rewrite them all to the same thing when other people press "Squash and Merge" on your pull requests -- and potentially a doxxing security risk.

I have considered re-reporting this to GitHub via the newer community discussions or via support again, but given the extent to which they've ignored all such reports over the last five years it is hard to find the motivation to do so.

It's definitely very dissapointing that GitHub will just change the commit metadata like that. I'm not strongly tied to GitHub right now so I might end up looking somewhere else for similar services.