Was it OK before fuel pump replacement?
If so then something is wrong with the new Sender in the tank or if you just changed the pump then old Sender was damaged.
The Sender for the fuel gauge is inside the tank at the top of the Fuel Pump Assembly.
Sender is attached to the Float in the tank.
The sender is a simple variable resistor like a light dimmer or Volume Control.
As the float rises up it moves Sender for more resistance, so gauge shows higher level.
As float goes down so does resistance and Fuel Gauge goes down.
In the dash board there is an "anti-slosh" module, this prevents the fuel gauge needle from swinging up and down as you go around corners and the gas in the tank sloshes back and forth, so there is a delay in exact level.
You could simply have a loose connection at the top of the gas tank, vibration is causing intermittent contact, so resistance varies from "real" level in tank, to no connection, which reads as very high resistance, so Full on gauge.
Delays are the anti-slosh circuit doing its job