It could also be caused by high filament flow.
It's easy to exceed what the heater can keep up with, using a standard 40W heater element and any one or more of: high print speed, thick layers, large nozzle, huge part cooling airflow.
If the flow is just above the max capacity of the heater at the selected fan speed, then after a couple of layers it would be expected to see the temperature drop despite heater pwm at 100%.
Because the slicer usually starts with a slower print speed on the first layer and then increases it for each layer, for the first maybe four or five layers.
Just like it does with the part cooling fan speed. It's to improve bed adhesion and reduce warping I think.
There is a setting in prusaslicer to limit the flow called "volumetric flow" or some such, but since I use cura I'm not yet blessed with a similar setting.
So I have to manually make sure the flow won't exceed 25 mm³/s, and that the heater block has a silicone sock, when I print PLA using a very powerful part cooling fan.
Or my weak heater won't be able to keep up.
A thicker, home-made silicone sock on the heater block may help with the heat loss due to the part cooling fan.
I'm going to try it out some day. Or maybe buy a 100W heater. Or both.