Found this while working on D15101. Cycles would incorrectly not update the ATTR_STD_UV attribute, because it existed on the old and new attributes list, but with different names.
That happened because the new ATTR_STD_UV was added first, without removing the old ATTR_STD_UV, so there were two attributes of that type. Since the code is always doing a linear search to find attributes, only the layer that was added first was found.