I was playing in my singleplayer survival world and attempted to set up a portal to link to my cave spider farm. I entered the nether built but did not light a portal at the coordinates to link and then returned to the overworld through a different portal to get more materials. Upon returning I discovered that the portal frame I had built had disappeared. I then placed some more blocks in the chunk and left and returned, again theses disappeared. I placed more blocks in the chunk and outside it to try again but this time upon returning to the over instead of spawning in the portal I spawned midair and dropped to the ground. I looked around and realized all the chunks had been randomly mixed together. I went into spectator and investigated more. Screenshots can be seen below. As can be seen in the screen shots tile entities were not rendering immediately. Otherwise mobs seem to have disappeared altogether. I have no backup of the world and it appears unrecoverable which is unfortunate so I hope this does not happen to other people. I am running some very basic client side only mods that do not directly affect gameplay to my knowledge.