Unable to launch automations via NFC tags on iPhone 11
Turns out I was in Airplane mode, and the NFC chip is not enabled in the background when in Airplane mode.
The reason other NFC functions work (e.g. write, Apple Pay, etc) is because there is an explicit NFC foreground UI present, which turns on the chip even when in airplane mode.
Unfortunately, this means you cannot have an NFC tag turn off airplane mode, for example.