I’ve been exploring, naming and placing banners, then using a map on them held in my main hand to create map markers. I noticed when I logged on after Realm updated to 1.13.1 that the most recent banner I map-marked was gone from the map. I logged out next to it, so I just put it back on the map again. Then as I explored, I set two more markers, and when I set the second I realized he first had gone from the map. I was unable to locate it again - as I looked for that one, the second new marker disappeared from my map. I found that one, placed on the inside of an igloo wall. It still showed a transparent banner hit box but the banner was invisible. I used my map on it and got an unlabeled map marker (the invisible banner had been labeled when visible). Since I could tell the banner wasn’t present and invisible, I thought that might have been why I couldn’t find the earlier banner I placed. I decided to try logging off and back on to see if the banners were visible again. 

Upon logging in, I was back at spawn with an empty inventory and no XP, as if I had just joined the Realm. I walked closer to a tall pillar we have built near spawn, confused. I logged out again, and back in, several more times. Each time, I was back at a random point near spawn, and not the same point where I had just logged out (for example, log out next to our pillar, log in far from it again). 

There are three other people on the Realm; I have not had a chance to have them test it (and I’m a bit afraid to after my experience). I am not the Realm owner. 

Update: I have been able to have another Realm member try logging on, mining/building for a bit, and logging back off and on - he is also reset at spawn as I was.