Should you change reference data in an API end point - rest

Question

We've recently started creating API endpoints. One of these end points is hardcoded to change 2 of our reference type codes (i.e. code: "P" for mobile is being changed to "M") from their system value to a custom value (out of a configurable list that has approximately 12 records at the moment. I'm trying to convince them it's bad practice and a terrible idea to change this reference data because of all of the issues it can cause for systems that use the api, however they believe it increases the "independence" of the API from the system of truth. We work in an enterprise environment and currently only our systems hit the api.
Is there any other data or information (Copious amounts of google searching hasn't revealed anyone discussing this sort of issue specifically) that suggests this is a bad idea? Or am I wrong in thinking so?
Edit:
For reference here's some examples:
What the data would look like in the source system the api pulls from
{
"phone_type": "P",
"phone_number": "1234567890",
"user_id":"username"
}
What that same data would look like coming from our API now
{
"phone_type": "M",
"phone_number": "1234567890",
"user_id":"username"
}
What the reference data would look like coming from our reference codes end point
[
{
"code": "P",
"description": "Mobile Number",
"active":"true"
}
]