Partially-made-portal when stopping ender dragon spawn in the middle of it

The bug

When you respawn the ender dragon with the new end crystal items and then kill them when they are doing the actual respawning you don't get a new ender dragon or a complete portal back, only two portal blocks

How to reproduce

1. Place all but one end crystal to respawn the ender dragon
2. Place a piston facing towards the portal two blocks away from where you would place the last ender crystal
3. Place the last end crystal and power the piston