AGOL and Portal don't handle contingent values, or even Domains in the same way as a database. Sure, you can publish a layer with a Domain to Portal but once it arrives it gets turned into a list. That list is then only applied to that feature layer. If you published multiple layers, they'll all have the same named List but they aren't linked anymore. If you update a value in a list for one feature layer it won't flow through to the other lists/layers.
You also can't add a layer from Agol or Portal (with a list applied) to Pro and then edit the 'domains'. They're not read only, they don't even show up.
