I don't believe this is correct, at least it isn't 100% of the time. Have no idea about UA/AC, but when US was still in *, my TT information always made it to US if it was a codeshare booked through UA. I know this because, first, I always got precheck when flying US, and second, when checking in with US, there was no option to put in my TT #, which there is supposed to be when it is not already included.

That said, you can not get an electronic boarding pass if you have a transborder flight out of Canada - with any airline. I'm almost 100% sure this is because of pre-clearance there. Whether it is because of some customs law that they won't, that they don't want to invest in the equipment to do it, its too much of a PITA (all BPs are stamped as having been pre-cleared, and AFAIK, the airline needs to keep a part of the BP with that stamp which gets collected at boarding) or for some other reason, I don't know. Since UA flights out of YYZ are all transborder, they just aren't offered, period.

Also, you can not get electronic BPs on mixed carrier itineraries. I've never seen it with other carriers, but definitely not with UA. What will happen, however, if the other carrier is first, is that the e boarding passes will show up on the UA app automatically when you open it at a transfer point when the rest of the itinerary is eligible. For example, if I'm flying YYZ-ORD-CVG, I can never got electronic boarding passes, however, when landing in ORD, the ORD-CVG pass always just shows up.

I've also had similar sort of trouble with UA itself not being able to issue further UA BPs. I was flying CVG-ORD-NRT-BKK, with everything on UA except for ORD-NRT, which was NH. At check-in at CVG, and both at CS and the club in ORD, could not issue BPs for the NRT-BKK segment, which was the UA-operated flight (at the time). Had to get it at the transfer desk in NRT.