cc @mkristian

I guess I missed a step getting preinstalled gems into the dist, because only the "default" gems (those installed into stdlib dir) are actually present.

Oh, workaround...

In both JRuby 9.2 and CRuby 2.5, these gems truly are just preinstalled. If you install the missing gems into your JRuby 9.2 dist you'll basically have the same setup as CRuby. And in both cases if you're using them in, say, a bundled app, you still have to specify them anyway:

$ bundle exec ruby -v -e 'require "net/telnet"'
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]
Traceback (most recent call last):
	1: from -e:1:in `<main>'
-e:1:in `require': cannot load such file -- net/telnet (LoadError)

@mkristian Thank you!