I think this thread discusses your specific question: [FEATURE] Allow reference tags to receive backfill data from sources like MQTT tags
I’ve not had a chance to check in on this behavior, but there I’m pretty sure there was a point in time in the past where the history was only on the base MQTT tag, requiring some custom functionality to indirect over to it for history queries (associated with your UDT). Last time I worked with this specific scenario was back in the 8.0.x days (and associated MQTT module versions of that time).
EDIT: just for clarity, the situation I mentioned above had History enabled on the MQTT Engine tag (and not the reference tags in the UDT). That used to be the only way I could get the backfill working properly.