I know that being territorially adjacent doesn't disqualify it from being a kingdom, afterall we have the example of England and Scotland. But why was the Algarve seen as a kingdom while other Portuguese continental regions were not?
I think I answered this in another post:
Algarve was always a nominal kingdom. In other words, its name was put in formal documents and texts to show that Portugal had sovereignity over it, and not Spain. It didn't have different institutions, different laws, or anything like Scotland or Ireland had in relation to England, for example.