Pretty frustrating issue when none of the 3 issues are applicable. 

• You lose access to a paid feature the Zap uses (e.g., multi-step Zaps or premium apps) due to a trial expiration or plan downgrade.
  Payments haven’t changed, plan hasn’t changed, permissions haven’t changed.
• Multiple errors occur each time the Zap tries to run.
  An email notification will be sent if a Zap is turned off due to errors.
  No errors, as the original question stated. All tests run perfectly, the Zap turns off regardless. 
• You make a change to a trigger or action of the Zap.
  You will need to turn the Zap back on once you are done editing it.
  Old Zap stopped. Troubleshooting with no results. Copied a new Zap, same issue. Created a brand new Zap, tested it, all successful. Zap still turns itself off immediately.