I'm not sure what entirely is causing this, and unfortunately it's a pretty time-sensitive issue. Disabling/adjusting the opacity or visibility of one (one!) specific layer, moving its Z level, or removing it, just ... removes the transparent background, turning it solid black upon gif export. This is a problem because the layer in question is a blink animation frame, and makes the whole thing terribly flashy. What's up with that. Truly, genuinely, have no idea what the problem could be. I can't post the file/screenshots by client's request, so I hope this explanation is sufficient.