I do have a duplicate post. It was about how that happened. For swimming underwater, spamming the jump button while having the Elytra equipped may end you up having your hitbox to 2 blocks high. I witnessed a different step. In creative, I fly up, then used the Elytra, dived, and I quickly unequip it. That'll randomly get the hitbox to 1 block height.
Ryan Ewing
I do have a duplicate post. It was about how that happened. For swimming underwater, spamming the jump button while having the Elytra equipped may end you up having your hitbox to 2 blocks high. I witnessed a different step. In creative, I fly up, then used the Elytra, dived, and I quickly unequip it. That'll randomly get the hitbox to 1 block height.