It's not a force push under the hood. Under the hood a new branch is created (first patch set is /refs/changes/nnnn/0, second is /refs/changes/nnnn/1, etc.). From the end user perspective the result is quite plausibly similar to force push.

The branch model must have changed. I used to push directly to a branch that was the change number (not the revision number) and of course had to force because it was the same.

So what happens when you push to changes/nnnn/12 when revision 11 hasn’t been created?

At least the way our installation works we don't push to changes/nnnn/12, rather changes are pushed to refs/for/{branch} (often refs/for/master). This endpoint isn't an actual branch. It triggers gerrit to look up the nnnn for the Change-Id in the commit message and create the appropriate branch for the next patch set.