CSS is broken only certain admin pages Drupal 8.2.7:
Similar to this question: Why does my Site have a broken theme/style?

In my case only certain admin pages css are broken and this seems to only affect admin, not unauthenticated users. For example /node/add/article. In the head tag, I see the correct aggregated css listed a few times:
/sites/default/files/css/css_Va4zLdYXDM0x79wYfYIi_RSorpNS_xtrTcNUqq0psQA.css?onn5ym

But some stylesheets are not aggregated:
/?onn5ym

The common consensus is that the files where css are stored are not writable by Drupal, but I'm not sure this is the case because some stylesheets get aggregated. Or they are coming from another directory I do not know about? I've also tried clearing cache, no luck. Any other suggestions?
Turns out this was a permissions thing after all. The entire web/modules directory needs to be read/writeable by Drupal.