Okay, I think your having a caching issue. It appears that updating the sub field is not clearing either the ACF cache for the field or the WP meta cache for the field.
Going to be honest, I don’t know if there is a way to update a sub field and then immediacy get the value that was just saved during the same request (page load). Generally updating of fields and reloading of fields happens in separate requests. I’ve just taken a look at the ACF code and I don’t see any apparent way to clear the current value old value from the cache.
You might want to ask the developer about this one to see if he has any advice https://www.advancedcustomfields.com/contact/
