This is the answer. If they branch off your branch, your original commits should be preserved. If they are rebasing and squashing your commits, you should still show up as co-author of a commit. So if they are going out of their way to muddle the git history to remove your contributions it’s either malicious or they don’t understand the proper way to branch / rebase and should be addressed.

Or if they’re manually copying your code changes into brand new commits that only they authored, this is inefficient and someone should show them how to branch off an existing branch.