That could happen if the base branch has changed a lot since the last time you rebased against it. Git may make you resolve new conflicts that look similar to the last time you resolved them, but they are in fact new conflicts, as far as git can tell.
All it can take is one commit in the parent branch. If your branch has many commits because you’re a commit freak then your fucked.
Only if there are changes in the same files and on the same lines in both branches. And if you’re a commit freak, you should probably be squashing/amending, especially if you’re making multiple commits of changes on the same lines in the same files. The --amend flag exists for a reason. No one needs to see your “fixed things”, “changed things again”, “fixed it for real” type commits.
What I do locally on my branch is my own business.
Honestly, when doing a merge/pull request into the parent branch, that’s when you squash. You don’t need the entire history of a development branch in main.