I had the same problem . Solved it by adding
apiVersion: “2.5”,

Since the default changed from 2.5 to 3.0

@sdetweil just wanted to say that it worked for me to do that.

I am new with posting here.

@sdetweil

this was the solution i came up with.

eventTransformer: (event) => {
    const currentYear = new Date().getFullYear();
    const age = currentYear - event.firstYear;

    event.title = `${event.title} (${age} year)`;

    return event;
},

Chatgpt is my friend.