vielleicht löst das mal Dein Problem:

Detail blocks
When setting the DETAIL flag of a block, this block does not increase
the portal count of the level, as normal blocks do. Detail blocks can
be used for small details in the level, like rounded arcs or other
elements which would otherwise increase the portal count and building
time. Because detail blocks do not suppress the rendering of the
scene behind them, they should not be used for large blocks -
otherwise they will reduce the frame rate. Note that detail blocks
are only visible in D3D mode. They are not rendered at all in 8-bit
mode.

Leider funktioniert dieses Flag noch nicht.