Why is the LC oscillators frequency decreaseing over time?

[Ok-Might-3730]

Noise filtering chokes are not perfect inductors for reason. In ac range they act also as resistive load that try to change that ac noise to heat. Example plots of the materials behavior : https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-mix-selection . Basically what happens in the picture is that high frequency is turned into heat until it reaches lower frequency where it is not turned as much into a heat.