Looks like a bug. concurrent_maplist/3 hasn’t been changed in ages. I guess it is in some sort of deadlock. First step would be to use Control-C and then ‘b’ (break) to get a new toplevel and then threads/0 to see the state of all threads. If that doesn’t reveal anything one may need the C level debugger to see what is happening.
If it worked for 8.2.4 it is most likely just luck and there is some timing or concurrency issue.