Project Properties Different - Addon vs Postman REST Call

I am getting Project properties via the /project/projectName/properties/propertyName API.
However I have noticed that my application (Spring, Atlassian Connect) receives different values from a PostMan REST call (basic auth, API token). It seems like an older JSON value is returned with the PostMan REST call, these values are not shown when the app is making the call (via RestTemplate, atlassianHostRestClients.authenticatedAsAddon).
Is this expected behaviour? How do I replicate the app connection in PostMan to see consistent return from the properties GET?

THANKS

@stevemac Might be related (or not), but we have observed the same problem in Confluence: Duplicate keys in content property .