Skip existing may skip ones that don't actually exist

My unit tests say the code is working fine but this user says there's an issue:

https://wordpress.org/support/topic/custom-image-size-has-red-x/

Start with the existing unit tests for the skip-existing behavior and compare their assumptions with the linked WordPress support report. Trace the code path that decides whether a thumbnail exists. Done means the reported red-X case is reproduced or ruled out and the unit tests clearly cover the relevant behavior.

Written by the indexing model from the issue text.