The fix described in this github issue did the trick:
https://github.com/elliotcondon/acf/issues/743
To apply the fix to ACF Pro 5.8beta3, change the following portion of pro/assets/js/acf-pro-input.min.js:
addSortable:function(t){this.$collection().sortable({items:".acf-gallery-attachment",forceHelperSize:!0,forcePlaceholderSize:!0,scroll:!0,start:function(t,e){e.placeholder.html(e.item.html()),e.placeholder.removeAttr("style")}})
becomes:
addSortable:function(a){this.$collection().sortable({items:".acf-gallery-attachment",forceHelperSize:!0,forcePlaceholderSize:!0,scroll:!0,start:function(t,e){e.placeholder.html(e.item.html()),e.placeholder.removeAttr("style")},update:function(t,e){a.$input().trigger("change")}})
