Problem with changing custom Hanami Mailer namespace

I generated mailer with command from documentation:
% hanami generate mailer welcome

Then I changed mailer a little and improved test. I didn’t change files location.
Result is here:

Test passes.

Now I want to put mailer in new namespace.
New mailer is here:

Test fails with error:
Failure/Error: expect(mail.body.encoded).to include('hello, world')
expected "" to include "hello, world"

So, changing default namespace in mailer leads to empty mail body. I looked deeper a little and found out that this happens because no templates found in lib/hanami/mailer/rendering/templates_finder.rb:86
Method template_name returns weird result.

Any help on how to use namespaces is highly appreciated.