I think I found a workaround for this problem. At least it worked for me and it worked for all my multi-image products.
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Go to Commerce Manager
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Catalog
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Items
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Check Mark the item you want to add images
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Click Edit
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Click the image of the variant you want to add an image
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Delete the first image so that there are no images
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Re-import the image you deleted. Hopefully, you have it saved somewhere.
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Add your additional images
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When you’re done click Save
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Repeat this process for all the variants you want to add images to (unfortunately)
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When finished, click Update Products at the very bottom of the page
Now when you click on a product that has multiple images you should see left and right scroll-through arrows.
Simply adding additional images to product variants didn’t work for me no matter what I did. But now it works. I tested this on both desktop and mobile.
Note: If you edit this product in Shopify and sync it to Facebook, you will need to repeat this process for that product. Shopify will override your Facebook product.
Hope this helps