Steps to Reproduce:

1. Go to the end, kill the ender dragon to spawn end gateway structure and go there 

2. Place a dispenser in front of the end gateway block and put a water bucket or a lava bucket inside the dispenser

3. place a lever/redstone torch etc and you will see that the water/lava block will replace the end gateway block

Observed Results:

End gateway block is replaced by the water/lava block

 ** 

Expected Results:

Even the dispenser is activated the water/lava bucket should not be used in this type of block