This has nothing to do with flat feet. If it would, you would tilt your ankle inwards since flat feet are about a lack of supportive structures on the inner side of your feet. What you experience is a tilt outwards, which does tell me that you actually lack weight on your foot and, thus, it is about your technique and not your medical condition.
I would say there is a difference between good technique and properly performing high kicks.
You can develop good technique practicing with lower kicks.
For example, a side kick involves specific movements, from chambering to the final leg extension.
These technical movement can be trained at the height your comfortable with. That could be at knee height.