Prepopulate plugin does not have return in the function of hookSaveItem and hookEditItem so others plugin that try to hook after him won't work.
As a general question
Why that rule has been set? It causes problematic dependence between plugin.
How i see it, it is can cause problem if the default is to block. Because new forgot is meaning "take". And more than that today it is not remove the plugins that run before him.
Should be an option that if it is 'true' it will "take" the event.
Or we can just make sure it's clear that you have to return true. I think that's the real problem. It's not clear.
You are absolutely right,
But how much it is costs to debug it after a forgotten moment.
Ok we'll fix it!