Iirc, this was a display issue one morning with United's website and was corrected afterwards.
That could be however this seems to be unrelated
On a round trip fare booked on united.com operated outbound by LX and return on UA there are 2 fare basis codes, one based on the LX change policy for the outbound and a second based on UA for the return
Additionally they notify you in the confirmation email
Changes with fee (this is because baggage and fare rules follow first TATL carrier policy)
Another thing that is more technical is that if you look at the actual reciept of a mixed itinerary like this it often displays as if UA uses your payment to purchase a second ticket issues a different eTicket number with payment marked as an EMD (often these reservations don't process immediately for this reason)