The problem occurs when you import a UDT that uses a scan class from the original project that has not been created in the new project. The tags are trying to use the scan class that is not defined so none of the tags (using the undefined scan class) work. That is expected but I feel like Ignition should let you know why your tags are not working. The fix for it is as easy as making the scan class in the new project or setting the scan class for the affected tags to a scan class that exists in the project but this affected one of our project deployments.
Please Fix This