Yes, butterfly bushes can die back. Mine has been in the ground for 5 or six years and this past winter was the first time it didn't suffer significant dieback. Every year before this, I had just pruned back to the ground, as there wasn't a lot of live wood left, but this year I have 3 feet of growth that survived so I'm only lightly pruning out the small amount of dead stems. Another reason for pruning back is that you get more
flowers on new growth and pruning to near ground level gives you more
flowers lower on the
plants without so much bare stem.
I think it would be okay to have the bulbs there as they would bloom before the leaves came out. anyway