I did not think of comparing the IP address returned from Unbound. However, I checked the servers’ names my devices were communicating - they were consistent with or without Unbound.
Main reason it took me so long to pinpoint the cause was reported issues like this which made me believe it was Apple’s problem / screw up:
So, I wasted so much time with Apple support as well as my own environment and even my ISP!
My ISP still doesn’t support IPv6 but 6to4. At one point, I thought that could be the issue - it was not!
As mentioned, after all steps, I focused on my 2 Raspberry Pi configured as primary and secondary and Unbound and etc!
Here’s more details about my problems with App Store …
For example, I can log in to App Store (macOS Catalina). There, I can see what apps I purchased but when I click on its little cloud icon to download the app/s, I was getting that error! I had to log out / in a number of times to get them!
There, I was able to log in to iTunes (now called Music) and go to my account detail. When I click on purchased history and then click on “back” arrow to get back to my account, it was popping up the message that iTunes Store is unable to process purchases at this time … blah blah!!
I did NOT want to purchase anything!!
No problem on my iPhone or iPad Pro downloading apps!
However, my Apple Watch was showing that error when syncing apps from my iPhone!! My workaround was like power cycling my iPhone and Apple Watch a few times! Then, I learned if I disconnect my iPhone from WiFi, then it would sync apps to Apple Watch!
Anyway, what a mess and stressful time!
I need to give myself break for a few months and then try Unbound again! Hopefully, it’ll get resolved - somehow!
Thank you all for your time.