Not OP but from what I understand, the problem is that their code has to interoperate with the “private” service run by the “bad guys”; it’s basically a more usable front-end. So the bad guys keep messing with the backend just to break the open client.

The government devs are deliberately modifying server code after seeing what the open source code is using to identify their header. Guthub doesn't require an account to view code, so how could they ban government coders from seeing their code?

I assume they read the code that tries to parse the data, and come up with formats that don't fit. If the code is open, they don't need accounts to see it

they need one to contribute changes though

The system that implements breaking changes is not the open source system. The open source system attempts to be compatible with the proprietary system. That is what it sounds like.

They don't contribute changes. Server and official client is maintained by Stockholmsstad contractors. Open source project is alternative client, not full solution.

They're changing their own code, not the public code in question. They're not submitting pull requests.