I had this problem before, setup file in data/settigns but can't see it in game. Last time I cured it by fresh install. This has failed using the S Full today and I've reinstalled 3 times this month for the same reason.

Does anyone know the cause or a fix?

Bump.

If people send me setups in-game I can acess them, but not those dropped in same directory from Setupfield etc...

The setup file name goes like

LX6_setupname

sometimes the filename is missing the _ and LFS doesn't see it. You have to manually change the file name to fit the format seen by LFS.

I will have a poke through the setting dir & see if that helps...