Jsonformirc.mrc, the .mrc file which comes with the json4mirc addon does have an on exit event, the only thing that event does is call the following alias:

Code
alias JSONShutDown {

  ;; Insure the alias was called as a command
  if ($isid) {
    return
  }

  ;; Close all json instances
  JSONClose -w *

  ;; Close the JSON engine and shell coms
  if ($com(SReject/JSONForMirc/JSONEngine)) {
    .comclose $v1
  }
  if ($com(SReject/JSONForMirc/JSONShell)) {
    .comclose $v1
  }

  ;; Free the hashtable
  if ($hget(SReject/JSONForMirc)) {
    hfree $v1
  }
}

You said 'Whenever' in your original post, indicating that you have reproduced this many times, are you saying you can no longer reproduce the issue at all?
If you can no longer reproduce, could it be that you must be using the json4mirc addon in some ways before you can reproduce?

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