iPhone 8 Plus creates duplicate Wi-Fi connection with fake Mac address

On your iPhone that is connected to your Wi-Fi network........what happens if you......

Tap Settings > Wi-Fi

Tap on the blue 'i" next to the name of your Wi-Fi network

Scroll down and Tap On Private Wi-Fi Address

Make sure that this setting is Off

Power off your iPhone

Power off your Wi-Fi router

Wait a minute

Power up your Wi-Fi router

Power up the iPhone

Check your connections again

Do you still see the "mystery" connection?

Use a private network address on iPhone - Apple Support

“To help protect your privacy, your iPhone uses a unique private network address, called a media access control (MAC) address, on each Wi-Fi network it joins.

If a network can’t use a private address (for example, to provide parental controls or to identify your iPhone as authorized to join), you can stop using a private address for that network.”

Sounds like a great question to ask of the manufacturer or provider of the Router you use. My guess would be your router supports 5gHz and 2.4gHz, but we can't know that. Your internet service provide would likely know, however.

My GUESS is this is a back-channel for sending usage data to Apple.

Nope.