Has anyone gone from months of absolutely no updates straight to approval without any warning, and if so how common is that kind of sudden jump in status?

Yes, that actually happens more often than people expect. A lot of cases sit with zero visible updates for months and then suddenly jump straight to approved because USCIS doesn’t always post intermediate steps online. For many people the first real sign of progress is the approval itself, so while it feels shocking and unfair, that kind of sudden status change is pretty common and doesn’t mean anything unusual happened with the case.