As explored in #33791 and visible with a slightly modified version of the reproducer https://github.com/EnyaEnya/multipart-null, adding org.springframework.boot:spring-boot-starter-data-jpa to the project breaks @ModelAttribute handling as of Spring Boot 3.3.4, and I have been able to reproduce only by adding org.springframework.data:spring-data-commons:3.3.4 to the project.

The same project with org.springframework.data:spring-data-commons:3.3.3 or without Spring Data is fine.