It seems like Wither Head Explosions can end up destroying the Wither Rose and Wither Rose dropped item entity.
In addition, explosions may be aimed at places which would cause the Wither Rose to be destroyed. For example, often, when one hits a chicken, the explosion may be aimed at its feet instead of its head, causing the Wither Rose to be destroyed in the explosion.

You would expect the Wither Rose would be placed, or at the very least the dropped item to survive.

Xisuma has reported this in a video (4:31)

RayWorks reported a similar bug as well in his video (3:05) and also made the bug report
MC-148047

The following bug report, which more or less reports this same thing, was posted a while back, but dismissed incorrectly, I believe.
MC-138670

This may actually be an issue with all explosions, but I'm not quite sure.