Another interesting thing is when reading an NFC card that has just domain without path, it works as expected:
https://short.com -- Shows "Open in Safari" notification, expected behavior since there's no App Clip with this invocation URL.
https://short.com/ -- Shows "Open in Safari" notification, expected behavior since there's no App Clip with this invocation URL.
https://short.com/a -- Shows the CPSErrorDomain error 2 popup. There's no App Clip Invocations registered with this URL.
When checking these three URLs at Settings -> Developer -> App Clips Testing -> Diagnostics -- it shows the same information for all three URLs (it just shows the same as in the second Diagnostics screenshot I attached in replies above).
*Just a reminder that short.com is just an example domain here.