The tests fail with Facter 4, this might be duetest/integration/puppetlabs/services/jruby/jruby_puppet_pool_int_test.clj#L625] because Timeout class in Jruby leads to Facter::Core::Execution thread leaking.
The problem was that the puppetserver test checked all the threads from timeout library instead of the object ID so it detected Facter's timeout thread like a leak thread. The test was updated in https://github.com/puppetlabs/puppetserver/pull/2424 and Facter doesn't impact the puppetserver any longer.