[yodeli]

Ha! I found someone with the same problem!
(check here, you'll see I tell no lies!)

I am currently working on solving this with very poor success .

Currently, I feel that by consciously straightening my left leg before impact it restricts this slide a bit.
But this is not enough and puts a lot of stress on the knee...

Maybe it is related to the feet: weight not evenly distributed at address and/or your left foot rolling on its edge (Have a look at 7-17).
To avoid this, someone told me to swing with a ball under the left edge of my left feet.
It is interesting but it didn't help me to find the cause of the slide.

If you have a good idea you'll make both of us happy !