Devise validation errors not appearing in view - ruby-on-rails

Question

While processing a new user registration, the log shows an expected
Completed 422 Unprocessable Entity in 462ms (ActiveRecord: 18.7ms | Allocations: 62328)
ActiveRecord::RecordInvalid (Validation failed: Email has already been taken):
[...]
Rendering layout layouts/application.html.erb
Rendering acqs/registration_alt.html.erb within layouts/application
↳ app/views/users/registrations/_new.html.erb:19
Rendered users/shared/_error_messages.html.erb (Duration: 0.1ms | Allocations: 22)
The sequence is correct as registration_alt.html.erb calls <%= render 'users/registrations/new' %>
However the partial invoked in the form
<%= form_for(resource, as: resource_name, validate: true, url: registration_path(resource_name), html: {class: 'registration'}) do |f| %>
<%= render "users/shared/error_messages", resource: resource %>
the file /users/shared/_error_messages.html.erb defines:
<% if resource.errors.any? %>
<div id="error_explanation">
<h2>
<%= I18n.t("errors.messages.not_saved",
count: resource.errors.count,
resource: resource.class.model_name.human.downcase)
%>
</h2>
<ul>
<% resource.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>
Is not rendering.
The registrations controller has an articulated create method, where it has to create an associated records.
def create
user_params = params[:user]
initialise_user_with_municipal(params[:user])
if user_params[:mobile].present?
sanitize_spaces(user_params[:mobile])
end
set_user_login_name
build_resource(sign_up_params)
# resource.save!
save_resource_and_roleshopuser(resource)
yield resource if block_given?
execute_devise_methods
end
def save_resource_and_roleshopuser(resource)
if resource.save!
#roleshopuser = Roleshopuser.create(role_id: 1, shop_id: #shop.id, user_id: resource.id)
if params[:user][:municipal_other].present?
set_shopzone(resource)
end
end
end
Now I suspect the chaining of record creation might be a source of issues. The error in this instance is occurring at the user creation stage build_resource(sign_up_params) for the line if resource.save! of method save_resource_and_roleshopuser so the errors should be filtering...
What is missing for the errors to percolate to the view?