Something I’ve noticed was that it’s not possible to add reviewers for a PR if you don’t have commit bits, e.g. if you’re submitting a first-time contribution.
According to the GitHub docs:
Note: Pull request authors can’t request reviews unless they are either a repository owner or collaborator with write access to the repository.
I can’t find any mention of a setting that would control this, but I thought it might be worth flagging.