When editing an ACF repeater in the admin the only reason that ACF would not move a field is if for some resewn the value(s) of a sub field is not submitted when the post is updated. This usually happens when a field is hidden by conditional logic or if a field is disabled.
Another reason this could be happening could be the priority of your acf/save+post action. Is it > or < 10? Actually I don’t see this as the cause, but sometime strange things happen.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users