To preface, this is 100% vanilla Minecraft with no plug-ins or mods whatsoever, on the most recent stable version 1.16.3. The only thing I could've done to possibly create this situation was manually copying the files from the world's save under the .minecraft folder and pasting them into a separate folder on my desktop. I haven't even used the backup files at all.
This is a long video, but the initial bug happens at around 12:45. Under the f3 information, you can see the dimension is the End, but I am clearly in the Overworld. I am still able to collect the dragon egg and get the advancement for beating the dragon. It should be noted that it is eternal night similar to the End in this version of the Overworld, and you cannot sleep in beds.
Not shown in the video, I am able to return to my 'actual' Overworld by entering the nether in this bugged out End dimension and then exiting it. I can return to the bugged out End dimension by going into the initial end portal that caused this bug. When doing so, it appears to return to a specific state (my house wasn't blown up for example when I made a return trip). It should also be noted that when attempting to go back to the End Portal in the bugged out dimension, the world literally drops off into a void after traveling from coordinates (0, 0, 0) to around 500 x value or so.