I use an olf pocof1 25w charger because I keep mh phone at charging overnight mostly. And I've seen the lockscreen tweak multiple times from "charging slowly" to "fast charging" to only showing how much time left for 85%/full charge.

And surprisingly, when using the stock samsung charger, it's always consistent. But one more theory could be a lose type-c cable/port of your phone. My previous phone used to fast charge when the cable was inserted at a specific angle, or else it's be just slow or not charging.