Maybe this is just a case of where the hardware is only capable of driving the 2nd display with a more limited set of DirectX features, I know my old card refused to do proper accelerated modes on the 2nd monitor.

MAME did drop support for some weaker cards at one point, so given the feature still works for some people it might just be something along those lines.

You've still not posted any kind of -verbose output for the cases that fail in the latest version tho.