When debugging this, use the console command “PXVIS Collision”. This will show you the physx collision in the scene.
Here is an example:
In my DM here the cell site count is set to 500 per DM. This means that there are a potential total of 4500 fractures which is 2500 over the hard coded limit of 2000.
In my DM I’ve set the support depth to 1 so that chunks stick together. So in this image you can see that I’ve hit three of the meshes to cause some damage. This changes the initial collision of 1 box to 500 for each fracture chunk. Even though they’ve not broken free, they have been damaged and now represent their own chunk collision. This way when they receive enough damage they will break free.
There are two depths that can be created in UE4. Depth 0 is the base mesh undamaged. As soon as a chunk breaks free the fractured mesh of Depth 1 replaced the original.
So in your case, since you have 6 DMs that are 500 fractures, you have a potential total of 3000 fractures which explains why chunks are disappearing.