Why when I use Queue Mode for RSpec then .rspec config is ignored?

The .rspec config file is ignored in Queue Mode because knapsack_pro has to pass explicitly arguments to RSpec::Core::Runner underhood. You can set your arguments from .rspec file in an inline way.

bundle exec rake "knapsack_pro:queue:rspec[--format documentation --require rails_helper]"

See passing arguments to RSpec.