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MQTT Engine Custom Namespaces, configured to parse the payload as JSON, do not handle changing JSON structures automatically due to the way JSON payloads are parsed.
Consider these two message payloads received sequentially on topic "a/b" at MQTT Engine:
{ "level": "value1" } { "level": { "key": "value2" } }
MQTT Engine will initially create the tag with the tagpath of "[MQTT Engine]a/b/level" with a value of 'value1'.
Now, when the second payload arrives on the same topic, since MQTT Engine does not automatically delete tags, it will try to create the tag "[MQTT Engine]a/b/level/key" with a value of 'value2'. However, level at this point is a leaf tag, not a folder. So, the new leaf tag called 'key' can not be created.