LG Velvet is on Mint network but AT&T has it labled locked

Suddenly I could not receive calls and my internet was slow.

If you are still able to use mint mobile for text and data, the phone is not locked. The phone is not voice over LTE compatible with the network you are using.  If it originally was an AT&T phone, then the problem is probably an update on mint Mobile's network but now excludes your phone from its voice network.   

Since the FCC requires that all phones connected to any phone network be capable of voice calls, mint mobile would be required to remove your phone from its network.

Whatever the problem is it is not because the phone is carrier locked now 

And if the phone is truly an unlocked phone and not originally an AT&T phone there is no way it can become locked to any carrier ever.

The problem is between you and mint Mobile and your current phone. 

This is the problem. Mint Mobile sent me this and said there is nothing they can do. The problem is that any carrier I want to switch to won't accept my phone because they see this.

Hogwash.  

If the phone were legitimately unlocked from AT&T it cannot become suddenly relocked after working on another network 

You said you purchased it unlocked. Purchased from where/who?  

And if it were locked it would never have worked on Mint.  Would not function for any services on a different service provider. Not calls not text and not data.

You said data was "slow",  or is it no connection whatsoever? No data no text?