Further to this, I’ve seen with the various field detectors that the iPhone (12 Pro in my case) seems to attempt to read approximately 4 times and then significantly decrease it’s read/NFC power whist on the Home Screen.
I presume this is a power saving tactic as whatever is in the field and pulling energy from the phone would continue to do so, with no actual function. As pointed out, with an app which deliberately reads a tag (showing the NFC reading dialog on the screen) this step back in power does not occur.