You mean the feedback being pickup ?
Did you notice the pulse length only change where I’m within a certain range of resonance?
I felt the capacitor was sucking up the energy and I think that maybe it cause a input source impedance to be much much low 20uf at 10khz is 0,8ohm
I believe that improving the coil leakage inductance will reduce the kickback of energy
That can be done packing the coils together interleaved
Having coils on independent coil formers is not a great idea on higher frequencies I guess
Specially if not giving square wave but pulse