Folks think about this for a minute dblNTSC mode is just an NTSC mode designed to display at double the rate and act like a built-in flicker fixer. WinUAE does this automatically with its 640x400 mode so there is no reason to want to use this monitor mode in the first place, NTSC with the right dispay options does the same thing producing a non-flickering NTSC 640x400 mode..
1280x400 is a doubled (super sampled 640x400 mode).. If you look at the original hardware you might notice vertically you don't see all the pixels all the time.. My advise is to remove dblntsc mode because 640x400 interlaced doesn't flicker with the right options set in WinUAE either..
That's my advice anyway, it's not really a BUG...It's a chipset behavior..Running WinUAE displays in DOUBLE line mode flicker fixes the dislay anyway (scan doubling).
You can then go into Brilliance and just set the default mode to regular NTSC or PAL 640x400 (not doubled).
