I've never heard of this happening before, but I did have a problem unlocking a phone from an old provider many years ago. I bought an iPhone 6 from a provider about 4 years ago, but it was locked and part of a contract plan and I bought it at a reduced rate. A few months into the plan, I wanted to switch, so I paid to buy out the phone since I liked the phone. The provider told me it was unlocked, but it wasn't, so when I activated my new service surprise surprise. I had to call the old provider back and work the issue out. It ended up not being the iPhone, so just be sure to check all angles on this one. If you didn't buy the phone through Public, its likely the manufacturer.
