The spec of resolve-uri() includes the non-normative note:

If $relative is the zero-length string, returns the value of the base-uri
property from the static context in the first form (if the base-uri property is
not initialized in the static context an error is raised [err:FONS0005]) and
$base in the second form.

I think the last part of this (the second form) is true only if $base is an
absolute URI. We've rather avoided saying what the result of resolving against a
relative URI should be, because the RFCs themselves are not very explicit on the
point.

The author of test fn-resolve-uri-2 has his own theories...

I propose removing the first paragraph of the Note.