UPDATE: I found out that the issue is quasi-connectivity or whatever, and that it's not a bug, just an unintended feature.

I looked for a similar bug report, which I found, but it's almost 10 years old and for a 1.6 snapshot, link below is to that bug report.
(https://bugs.mojang.com/browse/MC-15208?jql=text%20~%20%22piston%22)

Periodically, while working on redstone, a piston will stay extended, even when there is no signal going into it. When I destroy the piston and place a new piston in the same block, the new piston still has the same bug. Other redstone components put in the same space as the broken piston don't activate.

I don't know how to reproduce it, but it only seems to happen when I'm messing with the redstone connected to the piston.

I've tried breaking the piston and then replacing it, breaking and replacing the redstone connected to the piston, destroying every block surrounding the piston, using a button to send a redstone update into the piston, replacing the sticky piston with a regular piston, changing the direction the piston is facing, moving the piston with another piston, logging out and back in, switching dimensions, etc.