Spring Boot 2.0.0.M7 with the web-flux starter.
Not sure if I just have a screw loose or what, but invalid locale strings ("en_US" instead of "en-US") or missing the header entirely seems to prevent any locale from being resolved in controllers and elsewhere. Supplying a LocaleContextResolver bean doesn't seem to help. LocaleResolver doesn't seem to be compatible due to version, it looks like.
I think I would have anticipated a null locale, instead of a 500 error, or at least a 400...