This bug is new in 2.7.4 or 2.7.5 and still exists in 2.7.8. It did not appear in 2.7.3.

Running:
echo "<A><B>AAA</B></A>" | xmllint --relaxng test.rng -

with the following two trivial RNG documents should validate. But it does not. 
It says:
-:1: element B: Relax-NG validity error : Did not expect element B there
- fails to validate

If you rename "myself" (in both define and ref) in one of the documents, it then works again.

-- test.rng:
<grammar xmlns="http://relaxng.org/ns/structure/1.0"
         datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">

  <start>
    <ref name="myself"/>
  </start>

  <define name="myself">
    <element name="A">
      <externalRef href="test2.rng"/>
    </element>
  </define>
</grammar>

-- test2.rng:

<grammar xmlns="http://relaxng.org/ns/structure/1.0"
         datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">

  <start>
    <ref name="myself"/>
  </start>

  <define name="myself">
    <element name="B">
      <data type="NMTOKEN">
        <param name="pattern">[A-Z0-9]*</param>
      </data>
    </element>
  </define>
</grammar>