Copy below block and save it as DiffAlphaMask.xml and use that technique instead. It renders just like diff material, just discards alpha mask.
DiffAlphaMask.xml
<technique vs="LitSolid" ps="LitSolid" psdefines="DIFFMAP ALPHAMASK">
<pass name="base" />
<pass name="litbase" psdefines="AMBIENT" />
<pass name="light" depthtest="equal" depthwrite="false" blend="add" />
<pass name="prepass" psdefines="PREPASS" />
<pass name="material" psdefines="MATERIAL" depthtest="equal" depthwrite="false" />
<pass name="deferred" psdefines="DEFERRED" />
<pass name="depth" vs="Depth" ps="Depth" />
<pass name="shadow" vs="Shadow" ps="Shadow" />
</technique>
edit: it’s a technique, not mat.