Documents are marked for regeneration if they're not writable

Hello,

I’m using Jekyll with a very large set of content files, including ~1500 files in a collection of _authors.

We’re using incremental regeneration to speed things up, however it appears that these files are always re-rendered. Looking at the code, I’ve found this:

Can someone explain why we mark something for regeneration if it’s not writable?

Our 1.5K authors are defined with:

collections:
  authors:

Which makes them not writable (output is not explicitely set to false), which makes them all regenerate.
Thanks,
Stéphane.