piston not being updated when redstone turns off

Setup: redstone powering a block underneath, powering a piston underneath.

Problem: Piston does not update when the redstone unpowers, but does extend when powered. This seems to only occur on even redstone strengths (2, 4, ... 12, 14).

The gif attached shows this on strengths 14 and 12 but working on 13. Speed was increased to decrease file size, but the game was running at normal speed.