Mipmaps failed with Texture-Flag 256

I´ve tested this on various Computers with different G-Cards and drivers.
If you set flag 256 ( store in Vram), Mipmaps does not work.
LoadTexture "test.bmp",1+8 works correct
LoadTexture "test.bmp",1+8+256 works not correct

Any ideas? Is this Blitz or the drivers/G-cards?

Shodan