I’ve heard of “quicky” oil change places draining the front differential of a Subaru instead of draining the engine oil, which usually results in a damaged differential and an engine with eight or nine quarts of oil in it.
Why they would add transmission fluid, assuming they had drained the differential by mistake, is beyond me.
It’s not unheard of for the seal between the differential and the transmission to leak, but usually not on such a new vehicle. Is the transmission fluid contaminated with gear oil?