Indeed, this seems to be a known problem:
The keyword for this is "autoload and Thread" - refer
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/4589
and it is also in the latest ToDo's - I just didn't recognize it!
Maybe a link from the ToDo's to a more detailed issue description would be a
help?!
I hope, Matz find's some medicine against illness #4589.

Jens