Error converting CloudEvent to Background Event for Firestore Trigger Gen2

Hello, I'm migrating an existing Gen1 Firestore Trigger to Gen2 via this tutorial https://cloud.google.com/functions/docs/calling/cloud-firestore

The gen2 function is deployed successfully however it fails on parsing an incoming request before even passing the data to the handler. I see in the logs:

INFO 2024-08-19T06:10:04.013460Z [httpRequest.requestMethod: POST] [httpRequest.status: 200] [httpRequest.responseSize: 8.34 KiB] [httpRequest.latency: 3 ms] [httpRequest.userAgent: APIs-Google; (+https://developers.google.com/webmasters/APIs-Google.html)] https://storage-config-shop-products-gen2-rdl5god7nq-uc.a.run.app/?__GCP_CloudEventsMode=CE_PUBSUB_BINDING
DEFAULT 2024-08-19T06:10:04.017480Z error converting CloudEvent to Background Event: unable to unmarshal CloudEvent data:

... all document fields
<bunch of document fields dumped>
bonusPercent, error: invalid character 'ß' looking for beginning of value
Error: unexpected end of JSON input, parsing background event:

The firestore document itself is a regular doc with sub-objects and with an array field. I've tried to simplify the doc just to be a plain map: the result is the same.

The same GEN 1 function is working fine with no issue.

Deployment:

I'm using functions-framework-go v1.9.0, downgrading to v1.8.1 doesn't help
The GCP function is uploaded with 'go mod vendor' approach, which allows to use replace directive in go.mod
go runtime 1.21

The function is registered as in the example:

func init() {
    // gen2 register
    slog.Info("register MyFunc as a cloud event")
    functions.CloudEvent("MyFunc", MyFunc)
}

func MyFunc(ctx context.Context, event firestore.FirestoreEvent) error {
 ...
}

I see register MyFuncas a cloud event message in the logs on runtime start

GoogleCloudPlatform / functions-framework-go

Error converting CloudEvent to Background Event for Firestore Trigger Gen2 #251