Light map on foliage actors corrupting after closing the engine

Hi All

I’m using two foliage actors to create some rubble in my level but I’ve come across the strange problem , I build the lighting and everything looks fine the foliage has light maps and all is good, until I close and open up the level agein, when I do the light maps appear corrupted as show in the image, my shadow map rez is set to 8 so I don’t think its anything to do with that and the skylight is set to static as I’ve heard that can course problems. anyone have any idea ?