Was just making a circuit with slime blocks and redstone blocks when this happened. I destroyed the block and replaced it, and also got rid of all blocks around the piston.

When I put a sandstone block underneath the piston after I took the first screenshot, it fixed itself.

Then, I re-built the circuit, and activated it, and it did the same thing, with the piston staying activated. I've included all of the screenshots from my experiment.