The resharding coordinator consumes the estimation of remaining operation time, reported by the recipients, to engage the critical section when recipients are about to finishing cloning and applying oplog entries.
However, the estimation does not consider the possibility of having no oplog entries to fetch/apply. When there are no oplog entries to fetch, the estimation algorithm assumes fetching/applying oplog entires will take as long as the duration of cloning from donors, and the estimated remaining time will never reach zero. This would prevent the coordinator from automatically engaging the critical section.
This ticket should improve the estimation algorithm (here) to account for the aforementioned situations.