Deleting an MQTT Engine tag will result in a loss of alarm and history configuration.
However, deleting MQTT Engine tags can be required in certain cases such as if UDTs are being propagated from the Edge and the UDT definition on the Edge has been updated. This requires one to delete all instances of the UDT under MQTT Engine and delete the corresponding UDT definition so that it can be recreated/updated at MQTT Engine for the Edge side changes to take affect.
There are a couple of options for persisting either alarm or history configurations and they are documented in the following How/To's:
Unable to Resolve?
If the troubleshooting tips did not help you resolve your issues, please open a ticket with Support making sure to include the MQTT Engine logs as appropriate.
From the Ignition Logs view, select the Download icon to download a copy of the system-name.idb file to your local file system. You will need to compress (zip, 7z or rar) this file before sending to Support.