If multiple handlers (plugins) are subscribed, the firs one is overwritten.

Funny, why does noone tell me about this .. seems like a very obvious bug.

It only occurs if multiple "handlers" (plugins) are subscribed on the same namespace.
E.g. if simpletest module is enabled, it will overwrite the first handler for the "Drupal/" prefix.
(yes it's Drupal/, not Drupal\, we use path fragments as keys, not namespace fragments)

Easy to fix, commit will follow.