I reported it (with a solution) earlier as an issue but has been ignored by the developers. you need to modify this part of mtlfx.c (I left there old code as commented out):

Code:
function fx_mirror()
{
	// do nothing if mirror is already running
	if (bmap_mirrortarget) 
		{ 
	//		my.material.skin2 = bmap_mirrortarget;
	//		return; 	
			ptr_remove(bmap_mirrortarget);																// *** correct mirror shift on multiple level loads
			mw_bmap_mirrortarget = NULL;
		}
	bmap_mirrortarget = bmap_createblack(256,256,888);

...


another issue you can see is the shifted mirror view thanks to bmap_to_uv in mw_mirror_water_init() that can be resolved by using for example mw_bmap_water_uv = bmap_to_normals(bmap_for_entity(my,1), 50); instead, or simply use a fine normal map and comment out bmap_to_uv.

Free world editor for 3D Gamestudio: MapBuilder Editor