Just discovered this problem, and coincidentally found the solution at the same time. 😃 Lucky me.
Problem:
Random parts are missing while exporting to Gif or other video file formats. Seem like Spine can't turn on automatically certain slot images when exporting, this causes parts are missing in the final output.
Solution:
The problem can be solved by selecting the animation (that wanted to export) then export to Gif or other video file formats