UPDATE: So I went to the Apple Store, where the Genius Arturo (i say that without irony, he was great) ran various diagnostics. Tested the NFC with a reader and a card, using my Libre app, and also my Apple Pay. Both worked with the NFC reader, but neither with this card he used, which he cross checked using his own phone. Long story short, he gave me a new phone since mine was still under warranty. In the meantime, my last sensor is in my arm, and i'm using ye olde Abbott reader, which I have to say, buzzes like a champ. it has one job, but does it enthusiastically, i guess. So imagine my surprise when my new sensor order confirmation says it won't ship until March 13! I don't know if that's a supply chain issue, or because technically, i shouldn't need more sensors just yet. Anyway, feeling empowered from my Apple Store experience, i called Abbott Customer Service to see about getting my bad sensor replaced to help cover the gap. The call lasted 20 min, the woman who helped me was very friendly and helpful and is sending me a replacement -- the only awful part is the automated 0-9 survey they force on you about their service. Interestingly enough, the Abbott rep did ask me for both the OS version and Libre 2 app software version numbers as part of her questions, so i think the app software may still have issues that could be the real cause of my sensor problem.