The described behavior was observed in Aluminium SR1, SR3 as well as Silicon SR1.

In the Web API, the body of example schemas for POST requests does not match to the request url. For example, for mounting a device, I can send a POST request to

https://onap.lab:30267/rests/data/network-topology:network-topology/topology=<topology name>

which is the top-level POST in the request group network-topology. The example schema proposed by the API is as follows:

<?xml version="1.0" encoding="UTF-8"?>
<topology xmlns="urn:TBD:params:xml:ns:yang:network-topology">
       (…)
  <node xmlns="urn:TBD:params:xml:ns:yang:network-topology">
         (…)
  </node>
</topology>

Sending this POST request produces an error message, because the enclosing <topology> tag is not a child node, while the next inner level of the example request, e.g. <node>, is a proper child node for the request url. 
It seems that the top-level POST url in the request groups goes one level "too deep". For a mounted NETCONF device, we could not send the whole <config> via the Web API because only child nodes of <config> were allowed.