Hmm, that’s odd, yes there may be an edge-case.
FWIW I tried testing a few scenarios with:
- uploading 5 images using the upload button
- uploading 5 images by dragging in 5 photos at the same time
- uploading 5 images dragging and upload button on edit post
- uploading images on throttled connection
and auto [grid] is working here on Meta.