Entering the nether for the first time in a world causes the player to take void damage under certain circumstances. 

Steps to reproduce:

1. Create a new world
2. Go down to y=-54. Any x and z coordinate will work.
3. Build and activate a nether portal. Make sure the bottom obsidian block is at y=-55. 
4. Switch to survival if you’re haven’t already.
5. Reduce your health to 2 hearts or below. I recommended sprint-jumping around until your hunger is lower than 18 bars (so your health won’t regenerate) and then use the command “/damage @s 18 void” to reduce your health down to 1 heart. 
6. Go to the nether.

Observed results:

You will die immediately upon entering the nether. A death message of “<player name> fell out of the world” will be displayed, indicating you died from void damage.

Expected results:

You shouldn’t die when entering the nether. Instead, your health before entering the nether should be retained.

Discussion:

I think this bug occurs because when the nether is entered for the first time, the player’s overworld coordinates are temporarily copied to the nether when the nether is loading. This is problematic as y=-54 in the overworld corresponds to the void in the nether, causing the player to take void damage. If this theory is correct, then this bug applies to any nether portal built between y=-40 and y=-64. 

This bug does NOT occur if the player has already went to the nether in the world before. 

An easy fix to this bug would be to make the player invincible when the nether is still being loaded OR calculating and using the nether coordinates before teleporting the player to the nether.