Why would a step function cancels itself when there are no errors

Another reason could be that it reaches a quota limit, like for instance if your state machine reaches 25K events in the event history it will get canceled.

In my case I tried to start an AWS Batch Job in my step functions using the arn:aws:states:::batch:submitJob.sync resource type but provided not all parameters as Strings. But the additional Parameters has to be a String to String map.

So in my case the failing payload was

{
  "FieldName": 2
}

but has to be

{
  "FieldName": "2"
}

This may not match your case, but this happens in step functions when you have a Choice state, and the Variable you are referencing is not actually in the state input.

The documentation says:

Note:

You must specify the $.type field. If the state input doesn't contain the $.type field, the execution fails and an error is displayed in the execution history.

but from my experience, you get a Cancelled state.

Have a look here for more info: AWS Step Functions - Choice