|1. Place a piston down. (Regular or sticky)
2. Power the piston so that it is extended.
3. Replace the machine part of the piston (Not the piston arm) with a beacon.
4. After this, the piston arm will disappear. It will not be highlighted when hovered over. But, if you break the block where the piston arm was, block breaking particles will appear.
Edit by SuperGeniusZeb:
When using /setblock to replace an extended piston's base, the piston head will usually just be removed. But when placing certain blocks that have block entities, the piston head remains, but becomes invisible. So far, I can confirm this bug occurs when you /setblock the following blocks: