The bug:
When the piston is activated, the beacon beam passes through it.
Steps to reproduce:
1. put an activated beacon
2. put the piston on the beacon
3. trigger the piston with redstone
4. a beam passes through the piston
Expected behavior:
When the piston is activated, the beacon beam should not pass through the piston.
This is expected behavior, as pistons are transparent blocks. See MC-130858 for reference.