Yeah that's normal.
It's a little complicated as to why but imma try.
The phone hits 80% and wants to stop charging the battery. It can either draw the exact amount of power it needs to run just from the charger (bypass charging) or it can fully stop charging, drop a percentage, start again in a loop.
From what game booster says, the phone is capable of bypass charging, but only with a 25w usb PD PPS charger (or higher wattage). That's the only charger samsung seems to trust to supply the correct power for bypass charging.
Since that's really specific, the 80% limit will most likely just yoyo between 79% and 80% to make the limit work with all chargers
