Put in a Cleared Instance with a random during solo play

Was playing from Waterfall lvl 2 (did a reset as i tried to join a game for Vaal but it was finished.) When I reached the ancient pyramid (roughly 15min later, was lolly gagging and cleared full map) I went up the stairs and was placed in a cleared instance with someone who i was not in a party with. Proceed to run through the rest and it was full clear with Vaal dead