using a different ValidatorFactory for each request, sometimes the validation works, this is, for a value that does not meet the constraints, sometimes it returns the errors correctly and sometimes it doesn't return errors. Also it fails more than not (fails means, does not return errors). No exceptions, no warnings. Caching the ValidatorFactory causes that if it works once it continue working, or if it fails, it continue failing.

Ok, so you are re-creating the factory. And for a given factory it works or not. This indeed implies a problem in the bootstrapping and constraint extraction phase. There might be an issue how things are iterated over.