Why would a iMessage i send go from read to delivered to read again?

I have both iPhone x Max and iPad Pro.

I just noticed today that I sent a message to a friend, he read it.

The iPhone will show 'Read' but iPad Pro still shows 'Delivered'.

Then a few hours later, iPhone will show 'Delivered' and iPad Pro will show 'Read'

After 6 hours, now both shows 'Delivered'. very unpredictable.

both devices has iOS 12.1 and is up to date.

This only happened to last message, I saw previous message still has 'Read'.

On what Apple product running what operating system?

We are not mind readers.