This is called fasciation and nobody understands why it happens. It will not harm the plant but if I saw it on any of my roses, I would cut it back to normal, healthy growth

It happens on my Veronicastrum too. Only on one shoot this year though. The flower comes out like a purple tongue. The plant is enormous now, quite out of scale with my garden......but I like it.

I think veroncastrum is particularly vulnerable to it