Why when I use Queue Mode for RSpec then FactoryBot/FactoryGirl tests fail?

You can use knapsack_pro binary instead of rake task version to solve problem:

# knapsack_pro binary for Queue Mode
$ bundle exec knapsack_pro queue:rspec

Other solution is to check if your factories for FactoryBot/FactoryGirl use the same methods as Rake DSL and remove problematic part of the code.

The use of implicit association task can cause a problem.

# won't work in knapsack_pro Queue Mode
FactoryBot.define do
  factory :assignment do
    task
  end
end

Workaround is to replace task with explicit association:

# this will work in knapsack_pro Queue Mode
FactoryBot.define do
  factory :assignment do
    association :task
  end
end