Invert doesn't work in most video modes

[SpitFire-666]

In my brief testing I've had trouble getting invert to work

1. The -i parameter doesn't work for me for any mode I've tested
2. F2 (invert colours) only works in one mode I can find:

640x200 monochrome: working
320x200, 4 colours: not working
640x480, 16 colours: not working
640x200, 16 colours: not working
320x200, 16 colours: not working
320x200, 256 colours: not working

Cheers!