The Thymeleaf 3.1 release states the following:
Thymeleaf 3.1 establishes a general restriction on the use of classes from core packages: java., javax., jakarta., jdk., org.ietf.jgss., org.omg., org.w3c.dom., org.xml.sax., com.sun.* and sun.*.
If you are using Spring Data Projections, the returned class will always be a proxy in the jdk.* package, which means you aren't able to use it in a Thymeleaf template.
I have added as simple an example as I could think of to demonstrate the issue. Simply run ./mvnw spring-boot:run and navigate to localhost:8080 and the console output will show the error.