How to choose the right frequency when measuring coil inductance with LCR meter
simulate this circuit &ndash; Schematic created using CircuitLab
You have a large coil with lots of turns, each turn has its resistance and inductance. Between each turn there is also a capacitance, this is because the wires are close with  each other, separated with a thin layer of insulator.
The LCR meter measures the phase shift voltage VS. current. At high frequency the capacitance becomes more and more dominant, so your LCR meter &quot;thinks&quot; the inductance is decaying.
You can see those effects, described in this paper.


That's a very similar scenario as yours. At low frequency, the inductance is dominant up to resonance point, then the capacitance is dominant. You can see also the resistance increase due to skin effect.