i think it stating that external USB mic attached just means that your USB to headphone adapter is detecting. dont think it has anything to do with your TRRS splitter detecting or not.
i’m assuming when u say headset mic u mean the mic thats built in to the phone?
obv i can’t say for sure but i would bet its your TRRS splitter
i did lots of research on these generic splitters in developing my own TRRS splitter for the OPZ
there are so many different ways they can be wired up even beyond the 2 most common wirings
lots of wacky ones out there to my surprise.
and even if its wired up correctly doesn’t mean your phone
will detect the mic input on your splitter consistently or at all
the TRRS 3.5mm cable wouldn’t make any difference because your TRRS splitter is splitting the connections on the TRRS to 2x TRS jacks (or even a TS jack for the mic input)