Ok, strange though, as the TV/Monitor has no idea wether the mode is 8-bit, 16-bit or 24-bit, all it cares about colour wise is the amplitude on the R, G and B lines, and these doesn't work in another way because of bit depth.
Sounds more like the timings defined for your 8-bit mode differs from the others, or some general issue with 8-bit modes on the Amiga side.