Yeah, I’ve been having the same issues as well. If you upload the very same image, it will disappear from the gallery. So I had to fetch images from the CMS for my galleries. It’s a better solution long term wise anyway, but when you get problems uploading images for one time use - that does hinder your progress.
Since you’re using them for a gallery of some sort, I would advise to use the CMS. It will also help you manage multiple galleries across the website.
You can read about it here:
How to use the CMS for gallery:
The video in “Multiple Image field for CMS” helped get the iteration through the CMS content to work