The performance decrease also happens during regular gameplay (offline)
Sorry for the late reply, but I found a rare igloo structure with the zombie villager in the basement. I tried the splash potion again and this time it worked. The world I was on when it wouldn't work was a transferred world for the legacy switch edition. So the bug might just affect transferred worlds.
This happens to me on my transferred worlds on my Nintendo Switch.
I found a different shipwreck and got the achievement after finding that one. It's still weird how all shipwrecks won't unlock the achievement though...
It happens to me in survival, when I place blocks like dirt, they'll disappear and you don't get them back in your inventory. This is very frustrating.
this bug also affects tamed llamas with carpets and tamed horses. Villagers are also missing their updated textures. They all look as if they did prior to 1.14