On Debian,
the test "should calculate the optimal cost factor to fit in a specific time" in spec/bcrypt/engine.rb fails on s390x architecture (giving 11 for the first and 9 for the second)
It is certainly specific to this architecture, and since the test is time-dependent, it depends on the charge of the machine.
https://buildd.debian.org/status/fetch.php?pkg=ruby-bcrypt&arch=s390x&ver=3.1.15-1&stamp=1596191745&raw=0
But I just wanted to point out this issue.