If you are using RestClient, the issue occurs from Spring Boot 3.4.4, or more specifically Spring Framework 6.2.4, due to the following fix:

https://github.com/spring-projects/spring-framework/issues/34439

MappingJackson2XmlHttpMessageConverter is added, and added before MappingJackson2HttpMessageConverter, so even if jackson-dataformat-xml was already in your dependencies, the default content type changes from json to XML.