Unexpected API response payload for paginated migration endpoint for jira/classic:customField namespace

We recently observed a failed migration on processing response from the following Jira Cloud API endpoint: /rest/atlassian-connect/1/migration/mapping/$transferId/page?namespace=jira/classic:customField

Previously we observed(and expected) only numbers as a keys/values in the items fields, like:

{
  meta: {…},
  items: {
   “10302” :”10070”,
    …
  }
}

, where 10302 and 10070 are numerical parts of the custom field Id of format “customfield_10302”/“customfield_10070”

Now we are getting something like:

{
  meta: {…},
  items: {
    “10302” :”Original Reporter”,
    “Original Reporter” :”1070”, // Can’t say for sure now if we are getting non-numeric as a key or as a value
    …
  }
}

What is the exact contract for items element of the response payload for jira/classic:customField namespace?

Please find the list of related documentation I found, but which is not specific about the contract: