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
I wonder if it may be connected to this issue: Export gif missing images on some frames · #337 · EsotericSoftware/spine-editor
where the solution is to move the images into a dedicated "Images" folder, and relink the images of the Spine file so that the Spine file is not among the images. This should allow the gif or video to be exported correctly.
Another similar issue for the selecting the animation first part: Incorrect gif export · #433 · EsotericSoftware/spine-editor
Both should be solved, but if you're using a Spine version older than the one in the comments, then you might still encounter that bug.
The json exporting output works perfectly normal, the glitch only occurs on the Gif and video file so far.
Everything else like previewing the animation in the canvas or inside the preview window while exporting all function normally.
*All the graphical assets are inside an image folder within the project folder.
