I have a layer in Pro that has five fields that are all configured to use the same coded domain. When I publish this layer to Online, it publishes with unique coded domains for each of the five fields. So if I want to edit a value or add a new one, I have to add it to all five coded domains in Online.
This idea is to add some extra checking when publishing from Pro to Online to see if the Domain already exists in Online and only creates a new Domain if it's not already present.