This issue is due to a change in Ignition 8.1.7 that causes our UDT definition lookup logic to return the wrong value. We have a fix for this issue in our latest Nightly modules for Ignition 8. You can download the MQTT Engine fix via the link below.
https://ignition-modules-nightly.s3.amazonaws.com/Ignition8/MQTT-Engine-signed.modl
Let us know if you run into any other issues or have any follow up questions. Thanks.