Steps to reproduce the bug:

Each step has an associated screenshot in the attachments.
Step1: Have a comparator detect the water level of a cauldron through a block.
Step2: Put a piston somewhere, so that it can push or pull the cauldron.
Step3: Power the piston. I expected the comparator to turn off because there is no longer a cauldron behind the block. What actually happened is that the comparator remained on.
Step4: Even if you destroy the cauldron and the piston, the comparator will still remain on.

It also works with budded pistons, sticky pistons and slimeblock contraptions