It looks like a cat or fox has been rolling in it...
The middle is all flat though the rest of the plant looks really healthy and is flowering but it looks unsightly.
I first noticed it a few weeks ago and was planning to prune it but I've noticed today the middle is actually brown so I'm thinking that if I cut into it it's actually not going to regenerate?
I'll post a picture of it soon.
It looks healthy but very unattractive.