This happened regularly on all my laptops, though, so it doesn't surprise me it was happening to people's phones. Reported percentage was based on design capacity instead of last-full capacity, and shifted so 100% is "Full". So when the percentage reaches right about the point where all that's left is the degraded part of the battery, it suddenly jumps to about 3%.
(Since figuring that out almost a decade ago, I've been using my own battery-monitor script to see both percentage and health, so it's easy to see what's going on and just how bad my battery has gotten. Maybe the default notifications are better now?)