We did experience this, last week. We went from 1000 pending proofs to 8000 overnight. Our issue stemmed from an unannounced fix they pushed to PROD last wednesday evening, which moved all pending proofs into the Approvals. When they first released this to R1, it was just proofs like a week old, then they 'fixed' it but it also added proofs that weren't in active stages yet. They ran a script on our instance, to close out proofs that were on completed projects to remove a bunch of the old ones.