I have an invisible piston bug in an iron farm on a server (java edition, 1.14.4, no mods, survival).
A single sticky piston is activated in a loop by a simple slow redstone clock. Recently, the piston has disappeared, and the block that it's supposed to push does not move anymore.
- Blocks cannot be placed at the location of the piston body
- Mining the location of the piston body behaves as mining the air (i.e. nothing happens)
- The block where the piston arm should be can be mined and blocks can be placed there
- Sand will not fall if placed just above the piston, but will fall through if placed 2 (or more) blocks above
- It is possible to mine or walk through the position where the piston body should be
- Placing a solid block below it and having sand fall from 2 block above, the sand will stop at the piston body location, but cannot be mined there. It is still a falling block, and I can walk and mine through it
- The contour of the invisible piston does not even appear as in some invisible blocks bugs
On the two screenshots you can see the location of the piston body, on the left of the wooden planks, and the redstone circuit (very basic).