Strange JSON payload appended instead of tool being called

Hey there. We saw a strange issue recently where the assistant, instead of calling a tool, includes a JSON payload we don’t entirely recognize at the end of a normal text response. The JSON looks like:

{
    "recipient_name": "functions.our_tool",
    "parameters": {}
}

our_tool is the name of the tool that should have been called but wasn’t.
Is this a recent bug? We recently changed to model gpt-4o so I guess it could be related to that change.

Thanks!