Me and my friends realm (3 person realm), have a city world on our realm which has been going for 2-3 years now. Its on a superflat world which was pre-caves and cliffs which was at around y=0-4, 0 being bedrock, 4 being grass, dirt inbetween. When caves and cliffs increase the negative y level for building, we had built a whole sewer system, pools, basements, and a lot of stuff under bedrock. So today (6/7/22) we load up the realm after it updated, and 90% of the stuff under bedrock is deleted, leaving scraps of our builds down there. Not sure the cause of the issue, but even backing the realm up to pre-1.19 did not fix it. Old realm downloads from pre-1.19 also have the issue, but ive noticed not all the same stuff is destroyed, but still a large portion of our builds. You can see from the pictures the remains, and this is very frustrating to have put so much time into these underground builds, just for it to be erased with an update meant to ADD stuff to the game, not remove. If you have a bedrock realm from pre caves and cliffs on superflat, check if you have the same issue. Not cool mojang, im paying you my money for realms.