Looks like we gratuitously capture exceptions and when -d (cli.debug) is enabled we can see that we have a bug in format:

jruby -Xcli.debug=true -e 'p format("aaa", nil)'

A workaround for now would also be to disable cli.debug but this should not be too difficult to fix.

Resolving as not a bug. MRI also does this:

ruby -d -e 'p format("aaa", nil)'

I think the confusing part of this is -Xcli.debug looks like a JRuby specific option when it is just a property longhand for -d. So if you enabled -d in the C Ruby runs you should see the same problem in your CI runs.