Hi Dannie.
I am having the same problem. I am using pro version.
User Frontend PRO stores also the images/attachments IDs, but in the same meta key, but separately for years. For example:
‘_image_gallery’ => 383
‘_image_gallery’ => 345
https://wedevs.com/docs/wp-user-frontend-pro/tutorials/woocommerce-product-image-gallery/
But with version 2.9.3 I noticed that now image fields saves data to database in serialized array ..So this change makes my most of sites stop working like product galleries. Unfortunately they dont write this core change to their changelog.
I send a ticket to them to help us to find a way to revert this with some custom code but they advised me this:
if you do the custom code for your personal customization in the old way, then you have to change it again. ??
So i will take try to take care of myself.
And so i think your problem is for this update.
Good luck. ??