Hyrum's Law
With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody.
I'm in a team where all the consumers are only other internal teams and yet this still happens. I've found that it doesn't matter if you explicitly state to not rely on a particular behavior in your documentation, clients still will. It's always your fault if you break it because "it was your change that broke this client" and "it was working fine yesterday".