The with(user(...)) mechanism is intended to associate the current user with the HttpServletRequest and not the SecurityContextHolder. Since the user is not associated to the SecurityContextHolder the @AuthenticationPrincipal will not be resolved.
The reason it worked before was a side effect of SEC-2935 being broken. Unfortunately, this means that the behavior before was unexpected and not supported.
That said, you can still support your use case with only slightly more effort. If you want to use the with(user(...)) mechanism you need to ensure that the SecurityContextPersistenceFilter is registered with MockMvc. For example: