We identified the issue. Atlantis was using the wrong strategy for planning and applying terraform configuration, resulting in branches that had not rebased on the latest main branch to apply out-of-date updates.
We are addressing this by ensuring that atlantis uses the right strategy for applying plans that involves planning and applying the terraform config after merging the changes with the main branch (referred to as the "merge head".
We mitigated the issue at about 1pm PT, but will continue to monitor closely.