Different results returned when ignoring Primary Key index. Daylight Savings Time related bug?
I've got an answer that I don't like :


  Indexed searches convert the "value or range to be matched " into a
  UTC time to be compared against the indexed value.  
  
  Non-indexed searches convert the "stored value" from UTC to the session time zone
  for comparison.


That explains why I get different results with and without an index.  To me it feels like the implementation is broken, but I don't know what I would do for an easy fix. /shrug

Fortunately we can work around the issue in this case, but we are reconsidering how to handle this in the future.