Failing to handle two images with a total more than 256 palette entries gracefully seems to be a bug. Yes, you need a non-zero delay between images to get separate frames. According to the MNG spec, a sequence of images with zero delay will be combined into one frame. But the " image->next for ...
I'm having trouble converting some png files into a single mng: Apparently putting 2 Pallete type images together that have more than 256 colours in total fails: convert rose: -type Palette rose.png convert rose: -negate -type Palette neg-rose.png convert rose.png neg-rose.png rose-and-neg.mng ...