Featured Image Module Randomly Disappears

It is extremely unlikely that code file of identical content would behave differently.

From your description of situation I see two possibilities:

  1. File does get changed for some reason, you should not only upload clean copy, but actually compare what you have on server at that point against what you have locally
  2. You are mis-identifying upload as the key fix, it might be just related to upload or even completely coincidental

In your situation I would add some debug code (discreetly for admins) that would dump the current state of $_wp_theme_features global to see if it actually changes from expected, possibly during different stages of admin load.

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