I have seen bristleworms attack sun corals during feeding time. I suggest you trust your direct observations more than assertions made by random people on the internet
This does happen.
"But they only attack when the animal is dying anyway" this will be the next argument from the bristleworm police. And that my friends is what we call a non-falsifiable hypothesis. If a worm attacks an animal they will claim it was dying anyway, but there is no way to disprove this. Its a baseless assertion that cannot be tested.
I wish I had a solution for you. Not everything that lives in saltwater aquariums is compatible. I've seen bristleworms attack sun corals multiple times, and since there is no really effective way to eradicate them my only solution has been to stop trying to keep sun corals
