well!

deleting line:

version         ['v2', 'v1']

from api.rb and moving version block into:

class V1::Domain::API < Grape::API
  version  'v1'

  .....
end

class V2::Domain::API < Grape::API
  version   'v2'

  .....
end

Fixed my problem 😄

Right, the first syntax is not something that actually exists, it cannot guess that v1 is actually the V1 namespace by convention.

If you think the documentation is confusing, please do contribute an update. Because the syntax on the first case is actually correct, and says the parent API supports both v1 and v2 I don't think we can do anything here in code.

@dblock : right, I think we can update documentation for this.