I swam up into my end portal from below. I don't understand why that's an issue but I've since been able to recreate the bug by doing it again. It only works swimming up at full speed with depth strider boots.

When I spawned in the end, I was in the island, inside the blocks. I had to dig my way out so I wouldn't suffocate. Then I realized I wasn't on the central island. I had spawned at -1212 38 960, in the outer end islands. These are the exact coordinates of my end portal in the overworld, interestingly.

I had been to the end before in this world and this had never happened to me. I attached some pictures of the water under my end portal. I was setting up a beacon under it, and swam back up the hole I had made, and that's how I ended up in the portal.

My seed is 1066561068752604996