I experienced a slightly similar issue.

 

For the past year I have had both a iPhone 7 Plus, and a iPhone X on my plan - Happily paying the interest free payments for both.

 

Last month, the iPhone 7 Plus interest free term ran out - so it was good timing for the XS launch - I placed my order, but was then told I could't have more than 1 phone on Interest Free.

 

When I responded back advising this hadn't been an issue in the past, I was advised you are only allowed one and my order was cancelled. 

 

Slightly disappointed, but can understand the policy - Just odd that I was previously able to do this without any issues.