The bug
When either a powered rail, activator rail or a detector rail is pushed onto farmland using a piston, it does not drop the rail or get pushed onto the farmland. This does not happen for regular rails, those drop when being pushed onto farmland.
To reproduce
Get either a sticky piston or a regular piston and place it down
Put a powered rail, activator rail or detector rail to be pushed by the piston
Place farmland so that the piston will push the rail onto the farmland
Activate the piston
Actually, this is not only the case for farmland, but for soulsand and air as well. See MC-152037.