Cannot log in with ID on only one iOS device

I have 4 iOS devices (iPhone 6+, iPad Pro, iPad Mini 4 and iPad Retina 4th generation) and 2 Macs (iMac Retina 5K and MacBook) all running the latest public betas respectively.

On each I have to alternate between two Apple IDs (one with US based account and the other in South Korea). The reason for these two Apple IDS / accounts is I have thousand of dollars of purchases made prior on my US based account but now live in the Korea where I have my current banking.

For my iCloud info all on the US based Apple ID. I can log in for all devices with no problem.

For my iTunes I have to alternate back and forth from time to time to access purchases made respectively from the two stores (US and Korea).

Currently for all devices except the iPad Pro I can log into both IDs / accounts with no problem.

For the iPad Pro my iCloud is logged onto the US ID / account and can log out next log in with no trouble. For the iTunes Store I can log in and out of the Korean ID / account but cannot log into the US ID / account. Everytime I get the following error "Could Not Sign In - Your Apple ID or password is incorrect". Note again that I can log in and out of the iCloud side for this same US ID with no problem.

Safe to say that I am using the correct ID and password given I have no problem and tried the same a dozen or more times on the iPad Pro. To troubleshoot tried a complete Restore as a new Device and the same problem. In order to rebuild I had to load the purchased apps and media purchased from this US account using my iMac iTunes. I can still login to the iCloud side for this same US account and no problem logging into the iTunes Store with the Korean ID. Also tried changing my overall password for this account but same result.

Again no problem logging into my other devices.

Problem is driving me crazy! Thoughts?