@fredriksundlof-se the problem you are seeing is probably because the other plugin’s fields are still active on the page and have the same name. Values entered into the fields for the other plugin are probably overriding the ACF values after ACF updates.
As far as your filter goes. It may be working, but again, this filter is probably running before the other fields saves values and it is saving values based on what is entered into those fields and overwriting what is done by ACF.
In order to make the change you need to deactivate the other plugin.
