> the app on the device does not run.

That explains an oddity I was experiencing.

Work uses Webex. I had work webex installed on my phone. My password changed on my account in the office, if i try to open Webex on my phone I would be prompted to re-authenticate which I would never do because it required 2FA and the token generator is on my laptop which I generally wouldn't have with me when using my phone.

However, despite not being able to open the app as my account, I was still getting full messages in the push notification for anyone who had messaged me recently while the app was functioning. Anyone new would pop up as 'Message From X'.

More details are documented in the ntfy docs: https://docs.ntfy.sh/config/#ios-instant-notifications