I think this is a bug, though I'm not sure where exactly.
According to the Guidelines, @xml:base follows the W3C spec, http://www.w3.org/TR/xmlbase/. That sounds good and right.
TEI says the datatype for @xml:base is data.pointer. But by the spec the value for an individual @xml:base value need not itself be a valid URI. It can be a piece of a URI. A full URI gets hierarchically assembled from the individual parts in each of the xml:base values in the tree.
So an individual @xml:base in a TEI document need not be a data.pointer, contra the spec. Only the complete string assembled from such pieces need be.
I think the datatype for @xml:base should be changed and a note added somewhere saying that all pieces must form a valid URI.