I've seen linesets thru duct numerous times.
Since the lineset was run through a hole in the furnace unit, and the front of unit is flush to the wall, that's the only place they could run the lines. no where outside the unit to run them sometimes.
designer of the trailer persons running the duct at factory screwed up and ran the duct under the front of furnace.
trailer duct is crummy anyway, look like hacked w/ scissors and tap ins beat into top of ducts, but at least they could seal the hole.