as a start I want to point out, that my computer is a bit slow and this bug happened to me when I wass playing on my 160 MB mc world. Basically I just built a new nether portal and went through it. Because my computer is slow, loading up the world took a while (the nether Generating World screen). But then I heard that a skeleton shot me (an arrow sound) and a few moments later, I heard a ghast shooting a fireball. The world still wasn't loaded so there was nothing I could do. Then I heard a portal breaking sound (I assume the fireball broke it) and a few moments later, the world finally loaded and I was falling into the void and died. I got used to the world taking a bit longer to load (I wasn't worried tho - I knew my maxed out netherite gear could tank that). In fact, this already happened to me once (but without the ghast breaking the portal) - I went through a new portal and when the world loaded, I was falling into the void but for some reason it teleported me back to where the portal spawned and I didn't die.

PS: Thankfully I had a backup from a few day before, but it still made me angry so I deleted the world, so I can't give it to you. I tried to recreate the bug in test worlds and managed to do it, but I think it's just weird type of lag that may cause death. When watching the vid, notice the y coordinates after the world loads.

PPS: sorry for the wacky quality

PPPS: My version is 1.21.2 (not sure if hotfix), because I don't have a launcher - I downloaded MC BE a long time ago from the Microsoft store