I've seen cases where people didn't pass their probation period without doing anything wrong. I've also seen people fuck up terribly and pass the probation period. It's all up to the decision maker, they don't need to have any reason or explanation of any kind to keep you or let you go.
How was your probation?
Not about Contentful specifically but so far I have seen only 1 person fail probation while I have been in Germany. From my experience its pretty unlikely unless you literally don’t do your job.