Verification builds:

1.2.5.15 Beta

Summary:

Prior to 1.2 observers detecting a piston head that it was powering would form a "clock". This was incorrect behavior since observers, normally taking 1 tick to detect and output a signal, would have had it's signal ignored (as the piston head would have been retracting while receiving the pulse, therefore ignoring it). This was fixed in 1.2, perhaps as a side product of the faster observers and the instant chaining observers. It has returned in 1.2.5.15, possibly as a side effect of this bug being fixed:
bugs.mojang.com/browse/MCPE-23947

Steps to Reproduce:

1. Build setup as posted in the attached videos.
2. Turn the lever on to "prime" the setup.
3. Turn the lever off.

Observed Results:

The piston retracts, but then extends and retracts ad infinitum.

Expected Results:

The piston retracts, ignoring the observer's pulse since it (should) be occurring at the same tick that the piston head is retracting.

Screenshots/Videos attached:

Yes

Notes:

This is a borderline case of bug/game design issue. It seems that to fix this, the observer clock linked in the bug report above must be broken, unless some special exception is made.