When you dismount any rideable entity (boats, minecarts, pigs, horses, striders) below y=0, when you're in water or lava, it will teleport you up to y=0, causing you to fall down, possibly to your death. This only happens when enabling Caves & Cliffs experimental toggle.
Step to Reproduce
1. Go find any big new cave below y=0.
2. Go find lava or water aquifers, make sure to have a big space up left to y=0, so you don't suffocate.
3. Try to ride a strider on lava, or row a boat on water.
You should be teleported down at the same level as you were riding.
You get teleported up to y=0, and you may fall down to your demise.