According to Server Selection, PossiblePrimary servers are effectively Unknown, they shouldn't be candidates for selection. But this case statement appears to make them candidates for primary reads:

https://github.com/mongodb/mongo-c-driver/blob/a4bcfffa9c7ffa2e541a3351ec26cfad417cbdcf/src/mongoc/mongoc-topology-description.c#L292-L292

May also need a new Server Selection YAML test to catch this bug in all drivers.