Upon installing the update and running an old superflat type world, having a large void beneath it(due to it having been created before the Cave Update was released, which expanded below Y level 0,) which me and my girlfriend had been building in, within the void prior to the 1.19.0 Wild Update, we found that everything in said void(below Y level 0) had been deleted, reset and replaced with air blocks. Blocks can still be placed and constructed in the void but all our massive creations occupying said space were erased entirely, just below the bedrock layer. The world was hers and we played together on seperate consoles, both PS4 Slim models.

She had been, as of late, having troubles saving her world prior to this occurance. When attenpting to leave the world via the standard saving and quitting option, the game would get stuck on the saving screen. The game wasn't frozen, but it was unable to save. This had her resorting to closing Minecraft entirely from the Playstation game select menu while it was saving. Closing the game this way caused it to rollback to the last auto save which usually had us lose a very small amount of progress but wasn't too incredibly significant. I only include this to provide as much information as possible, regarding things that could influence the bug. That issue may just be a hardware issue, specifically with the hard drive and not so much on Minecraft's end, although I will say that the Playstation version appears to be the least cared for version of Minecraft, having the most bugs and severely lacking optomizations, and I guarantee that said bugs and absences of optimizations significantly help in causing issues related to the hardware. The simple fact that an autosave works but an actual "save and quit" doesn't, adds evidence to that statement. Regardless, the issue itself is likely just an oversight.

I believe that code was left in that would add onto the world below Y 0, as it did for the Cave Update. Since the same code was there, it caused everything below to regenerate despite there already being things there, and obviously that would mean destroying anything below Y level 0, effectively reseting it. I don't know if this effected other worlds or how it would have because I don't believe I have any other worlds where I explored or built below Y level 0 to the point where I'd be able to check and notice a difference, and she doesn't either.

I don't know what the intention was for this occurance. Perhaps it was intentional as that was how it was planned to incorporate the Deep Dark biome(although this doesn't make sense as there was no warning given that anything built below Y level 0 would be reset), or it was purely a mistake, which is more likely, as I would think that if it couldn't be generated in unexplored areas beneath within an already ventured in chunk, it could still just generate within new chunks and not be all that much of a big deal, only requiring those of old worlds to venture out more to see the new content, which is how it's basically always been for new updates of that kind. It's possible that this issue only exists for superflat worlds but as stated, I cannot confirm or deny wether that is true or not.

Ultimately, this glitch destroyed so much of our progress, resulting in many hours of our time amounting to nothing. I was in the middle of building something, just approaching its completion, and now it will never be completed. My girlfriend had constructed many command block creations that have all been eliminated without a trace.