This appears to be a duplicate of MC-123363, the issue flew under my radar as it wasn't labeled as affecting the prerelease.

I created a datapack that generates custom ore using the following method:

1. In each chunk, a structure block is placed that uses integrity to randomly distribute command blocks in chunk.
2. These commands blocks run a function that checks the surrounding area for spawn conditions, and if those are satisfied, it setblocks an 'ore' (custom player head).

The crash occurs in areas with a lot of water, ie. oceans. I attempted to replicate the crash without using the ore generation system, but have been unsuccessful. It appears to be somewhat random, as the 'ores' can generate in the water without causing the crash.

To reproduce:

1. Install the attached datapack in a world
2. Run /reload
3. Find an ocean or area with a similar amount of water.
4. Fly around in creative a bit, should crash within ~30 seconds.

note: The whole datapack is very large, so I've stripped out as much as I can. Shouldn't make a difference in resolving the crash.