Piston never stops extracting and retracting once started

Put the summary of the bug you're having here

What I expected to happen was...:
Removing the redstone should just retracting the piston

What actually happened was...:
The piston did retract and directly afterwards extend and continued this in an endless loop

Steps to Reproduce:
1. Put a sticky piston so that it can extend vertically.
2. Put a slime block on the stick piston extension side.
3. Put a redstone on the slime block

4. Put a redstone on the piston. (Piston should extend moving slimeblock and redstone on top of the slimeblock and stay extended) (see attachment)

5. Remove the redstone from the piston. Now the motion begins.