False-positive breaking change when adding anyOf to a response-property-type-changed

[ashb]

Describe the bug

I have a response property of nextRunAt that was before defined as:

          "nextRunAt": {
            "description": "Time at which the next DagRun can be created",
            "format": "date-time",
            "title": "Next Run At",
            "type": "string"
          },

And now it's changed to the following:

          "nextRunAt": {
            "anyOf": [
              {
                "format": "date-time",
                "type": "string"
              },
              {
                "type": "null"
              }
            ],
            "description": "Time at which the next DagRun can be created",
            "title": "Next Run At"
          },

(It's a question if it should be anyOf or oneOf, but oasdiff does the same)

A clear and concise description of what the bug is.

To Reproduce
Sorry it's not a full snippet. Hopefully the above is enough detail.

Expected behavior

Since this is adding a new type that can now be returned (null) but the existing type was still there this should not be a breaking change.

[ashb]

Thinking more about it I guess this could be a breaking change, as if a value (a string) was always expected before, but it could now be null then the client could break.