Turns out this is a result of the green->transparent feature I added in 1.12.0--the code scanning for green in the global color table assumes there's a global color map, and this gif doesn't have one. I've added a check for that case, and I'll fix the green check to work with per-frame color tables, too.
And here's a copy of that gif with a global color table, generated by running yours through Optimize animated GIF with the Coalesce (unoptimize) setting: 