I have a registration form - from FOSUserBundle this is in the template:
{% form_theme form \'AcmeMyBundle:Form:errors.html.twig\' %}
Ok, this is a known issue.
Let's have a look here : https://github.com/symfony/symfony/issues/2605
The solution is : create your own validation group for properties' validation rules you want to override. In your validation.xml, put only properties you want some different rules, and define validation on a new validation group.
So, for validation.xml, to modify only plainPassword validation rules for your entity (Acme\MyBundle\Entity\User) :
And now you have to state that you use different validation groups for the impacted forms (registration and profile). Fortunately, FOSUserBundle is a good practices' example and allows you to override them in your config.yml :
fos_user:
registration:
form:
validation_groups: [Default, RegistrationAcme]
profile:
form:
validation_groups: [Default, ProfileAcme]