This is an existing issue that was closed prematurely.

https://bugs.mojang.com/browse/MCPE-41513

(edit: this is the duplicate actually https://bugs.mojang.com/browse/MCPE-43159

The realm owner reset the end in 1.12.0, so this gateway was not previously loaded and it showed a beacon beam when I loaded it. I built a platform around it and proceeded to throw an ender pearl into the gateway. The game never came out of the black screen. My coordinates indicated that i had made it back to the main island, however, i could not move, could not place blocks, and upon throwing another ender pearl it did not make the noise or teleport me anywhere. I attempted to relog, but now the game hangs on the loading terrain and i cannot join the realm. 

This is very unfortunate, as I thought the gateways had been fixed per the changelog. I thought that resetting the end would lead to all good portals and possibly better realm performance. 

The linked bug report has more information, but the moderator closed it. 

Edit: as I was the only one online at the time, after remaining logged out for 10 minutes, the server had reset itself. When I signed back in, I spawned on the obsidian platform, which is as intended. Since it was a distant end gateway, it was not linked to the end gateway that opened when the dragon spawned, and therefore I could not have been stuck in bedrock. There were no obstructions on the platform, so it did not need to regenerate itself (although that may have been the point of failure nonetheless). I had the plan to bring chorus fruit with me next time, but since I wasn't stuck in a block I don't believe that will work.