Hi Lex123,
We solve this issue for heaps of people at the beach who also have the same issue. You need the footstrap to be as narrow as comfortable, NOT as tight as possible.
Think about it, if the footstrap keeps your foot in through downward compression, when you impact a piece of chop, the footbed compresses releasing the compression holding your foot in place.
If the footstrap is tighter on the sides of your foot and just snug over the top, then you rely on kinking your feet, front foot at 45 degrees to centreline of the board and back foot at 90 degrees to the CL of the board. If your footstraps are ducked to around 22 degrees this will allow your foot to lock in place tight as, and still be easily able to get out by aligning them with the strap.
This only works on boards with multiple inserts allowing the straps to be set wider or narrower.
Have a go, I'm sure it will sort your issue out. If not, you need some lux footbeds and some new footstraps.
Good winds,