I am using 6.3.4, but if I send a post request to /server/collection/subcollection, only the collection gets created (and the document is added to collection, not to subcollection).
Could you provide us with an sscce that allow us to reproduce the issue?
In view of the above, could you please throw more light on how the subcollections can be created using REST and how documents can be added to subcollections using REST?
The documentation will be updated with the upcoming official release.
Best, Christian