I think LX is almost universally blocked.

I have a theory on it too. UA's blocking is a function of inbound revenue from carriers for UA award seats. Since LX doesn't have its own FF program and uses LH M&M, UA never gets any revenue from LX for award tix booked by LX flyers. Instead, LX flyers look like they come from LH, so UA has a stranglehold on the LX seats as a result.

I also bet UA has no idea this is happening.