Simply put, the ‘Do not fracture’ doesn’t work on the chunks.
I have icicles that you can break, and I want the base piece to stay stuck in the wall while the rest breaks off. No matter what I do, the entire piece of ice shatters and pops out of the wall.
In order for the chunks to be supported you will need to set a support depth of least 1. This will allow any chunks at that level to only break free when they receive enough damage to do so. If you want the pieces that are in contact with another surface or overlapping, like the base of the icicle then you would also need to enable the flag for World Support.
If you’re noticing that a single hit is causing all the chunks to break free and fall then you have four options.
Increase the the damage threshold
Decrease the Damage Spread
Set a Damage cap
Select chunks for the base and then in the Chunk Parameters tab set them to Do Not Fracture.