Exporting an animation as a GIF produces incorrect result when scale is large enough

[mitchcurtis]

The following results are for an image where each frame is 64x96:

good:

261x391 (4.08 scale) 262x393 (4.10 scale) 263x394 (4.11 scale)

bad:

273x409 (??? scale, no animation, white section on bottom) 263x395 (4.12, skewed animation)