I have been able to recreate this bug. I went to join my server (it is run on my network, I'm not sure if this has anything to do with it) and quickly canceled. This brought me back to the multiplayer screen where I went to Singleplayer and loaded a world from my singleplayer list. Upon joining my single player world I quickly warped to my location on my server and was able to move around (somewhat). At this point, I was loaded into both my server and the singleplayer world at the same time. It seemed as though I was able to move around but was restricted to the terrain in my singleplayer world. My screen showed where I was on the server. I was NOT able to move my player at all in the server but I could still interact with entities and containers if my player was within range of them on the server-side. There were a few other odd mechanics that occurred throughout the experience. I took a video after I realized what happened but I didn't get a video of how I produced this bug. I can provide more video or photo evidence of things that happened during the experience. Hopefully, someone can make sense out of this! *Note the images just show the mismatch between the server and my singleplayer world. The player was also running 1.15.2 and could interact with my player