Yes, but only in very limited circumstances. If you:

1. fork a private repo with commit A into another private repo
2. add commit B in your fork
3. someone makes the original repo public
4. You add commit C to the still private fork

then commits A and B are publicly visible, but commit C is not.

Per the linked Github docs:

If a public repository is made private, its public forks are split off into a new network.

Modifying the above situation to start with a public repo:

1. fork a public repository that has commit A
2. make commit B in your fork
3. You delete your fork

Commit B remains visible.

A version of this where step 3 is to take the fork private isn’t feasible because you can’t take a fork private - you have to duplicate the repo. And duplicated repos aren’t part of the same repository network in the way that forks are, so the same situation wouldn’t apply.