This happens, when the mast sits not perfectly in the box.

When You insert the mast tighten the screws, but not too heavy.

Then: push the mast forward with Your body weight, then tighten the front screw.
Push the mast back, then tighten the back screw.

Do this 3 times forward and back. Then the mast sits perfectly in the box and the screws should stay tight. Besides that is is also better for the screws, when You dont put too much strength on them, what is not nesessary with this technique.

Are both screws loose? How loose? Are you rocking the mast back and forth the mast while tightening? I usually do this with the fuselage connected so I have more leverage: rock forward and tighten front screw, rock back and tighten back screw. Repeat several times until you feel the screws are tight. Remember to apply a lot of pressure when rocking back and forth (you will definitely put more pressure when foiling).
dry silicone, like mclube on the mast head and foilbox can help a LOT.

Try it on the beach

^^ Could be a fun crash with a T30 shaped hole in your chest. I hope its very secure.

Yes, I remember him competing during the beginning of foil in the PWA. He should know some tricks :)
Thanks!