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.
