nfc

Are you using that image, or are you using something that you compiled yourself?

Yes

Are you taking the Puck out of NFC range before changing the tag with NRF.nfcUrl? I wonder whether that could be causing problems...

Yes, but perhaps my reader does not "finish" the transfer properly and the nfc state machine is left in a undefined state.

Do you have any contacts at Nordic who might be able to comment on this?

If not, I'm fine with keeping this behaviour in an unresolved state for the moment. Perhaps we can revisit it when you are not that busy. I haven't changed anything to the start/stop code. So mine should be fine too for most people. However it's a very long stretch as long as we do not know the reason for this anomaly.