After a little chat with the people at Spring i have now been redirected here as the problem is in the Thymeleaf part of the Spring java code. The links above contains all of the details but the short story is:
Any html template with th:field included and rendered through TemplateEngine .process will be using EngineContext to render it (not WebEngineContext), EngineContext will never create the nescessary SpringWebMvcThymeleafRequestContext for the field bindings to work in a form.
I found a "dirty fix" by creating my own SpringWebMvcThymeleafRequestContext and injected it in to make it accessible for EngineContext.
I am asking for an official fix to this.
Make it possible to use thymeleaf templates running through Spring TemplateEngine .process(..)
A possible solution would be to simply create an object of SpringWebMvcThymeleafRequestContext in EngineContext and use it to parse forms.