SERVER-34895 made it so that the stable timestamp would never advance ahead of the all committed timestamp, so that single node primaries never storage-commit a wiredTiger transaction behind the stable timestamp.
Consider 3 operations in flight on a single node replica set, at ts 1, 2, and 3. If 1 commits first, the all committed timestamp and lastApplied and stable timestamp will all be set to 1. If 3 commits next, then the lastApplied will be set to 3, but the all committed timestamp will still be at 1, keeping the stable timestamp at 1. When 2 commits, the all committed timestamp will advance to 3, but the lastApplied will not advance, and so the stable timestamp will not advance either.
This will lead to majority writes not committing until another write comes in.