I don't think this is isolated to UA either. I distinctly remember being routed through DFW and SFO on DL once each. I do recall having checked luggage on the SFO one and remember doubting the check in desk that it would be checked through to my destination. Lo and behold the app told me it was offloaded in the baggage claim and I hustled out and to check it in again and then back through TSA and onto my flight.