Hi,
It was originally designed for 50kHz switching frequency and then you changed it to 70kHz. Why do you have to change it in the first place? You changed only the frequency? What else did you change?
Here is the starting point of your problem. As you increased the frequency by 20kHz/50kHz=0.4=40%, you needed an inductance that is smaller in the same proportion. So using the same old inductor means you didn't underrated inductance. You had 40% extra (or excess inductance) and so what happens to the flux?