I added an image gallery with the Avia Layout Builder to a page and sorted the images manually and save the page. When I click a second time on the button “Add/Edit Gallery” to add more images to the gallery, my sort order is lost.
You have a lot of plugins installed. Maybe one of them is causing the issue. Deactivate them one at a time, see if it fixes the issue. I tested this on my end and the sort order is not lost when I added new image on the gallery.