Export As / Save For Web size limit conundrum. How did I export an animated GIF at 1200 × 12404px??

I have managed to export an animated GIF from Photoshop at 1200 × 12404px and can't replicate it.

I create eDMs for different brands almost daily. These eDMs often contain animated GIFs, and at least a handful of times a month, their height exceeds the seemingly arbitrary Photoshop limit of 8192px.

Yesterday, I created an eDM with two animated sections, exported it as an unsliced GIF and a JPG without thinking about the height, and sent off the previews to the client for review. 

This morning, the client came back happy and asked for the slices, so I went about my usual process of slicing and exporting. I checked the height and realised it was 12404px, and I'd need to split the file to export at the full width. For some reason, I also checked the height of the preview files and realised I've somehow managed to export them at the original size of 1200 × 12404px. How did I do it?? I must have used the Save for Web export because I don't know of another way of exporting animated GIFs, but why didn't it get resized to the 8192px limit yesterday, but it did today? 

I'd love to hear your ideas on this because I'd love to be able to replicate it.