How to reproduce:

1. Build a little tube with an opening like the one seen below (The block on top is just so it doesn't burn, so it's not entirely needed)

2. Summon in a phantom with No AI.

/summon minecraft:phantom ~ ~ ~ {NoAI:1b}

Make sure it doesn't suffocate when you summon it in

3. Punch it (it may take a couple punches for it go go inside of the blocks and not suffocate)

Alternatively, you could just try to push it into the back of the tube

When it goes in the blocks, it will turn completely black (apart from the eyes), and then it will revert back to the spot it was in.