Are the comments attached to the pull request, or particular lines of code or commits?
If they’re on the PR itself they should persist.
If they’re on a particular line of code or commit, I think they persist if the given line of code/commit hash is unchanged. Note that the commit hash can change if prior commits were altered, even if the commented commit itself did not change.
On a PR when someone comments on code/commits, the comment is automatically displayed in the PR comments as well - which is what leads to this confusion. In general, I encourage people to comment on the PR directly and link to the lines of code/commits as needed.
You can at least look through the comments before force pushing and if you identify anything that will be lost, you can quote the text in a new comment on the PR itself.
NB: this info is based on my personal experience which may be outdated.