I have encountered the same problem several times before. If my phone (s9) is doesn't have very high battery before work I'll switch it to ultra power saving mode in case I need it since I cannot use it while working. Every single time I turn ultra power saving mode off the battery drains within five minutes. I just tested it and within 2 minutes it went down from 8 percent to dead. This has happened every time I needed to extend my battery so has anyone else experienced similar problems? Anyone have any solutions?
Thats kinda extreme if you ask me, does it also drop this quick when its not in ultra power save mode on the last percentages?
If yes: Try to charge the phone to 100% and do not charge it again until it dies, repeat this cycle 3 times. This effectively recalibrates your battery. If no: In this case I would consider it a software bug.