Hi,
can you share the message you sent using template 101? One can state the name of the devices as third parameter: 101,uniqueChildId,myChildDevice,myChildType. This way a child device of type ‘myChildType’ with name ‘myChildDevice’ and external Id ‘uniqueChildId’ will be created.
For the root device it is “100,{{name}},{{type}}”
For child devices it is "101,{{UniqueDeviceID}},{{name}},{{type}}.
Now to the tricky part:
You have to make sure that either 100 or 101 for child devices are the first messages received by the platform before sending any other messages because of race conditions.
Most likely you do that by sending it QoS 1 or 2 and wait for the ACK.
It sometimes happens that you send 101 and in parallel already some data for the child device which will be processed first and therefor the device created with “MQTT Device …” and 101 ignored.