So seems SfButton won't render properly when it exits on a page under the hiearchy of AbsoluteLayout.
I tested on a separate Page with StackLayout and it worked so it seems likely AbsoluteLayout is the culprit (I also tested in on a few places on the same page)
So I was mistaken, the problem was apparently that I had a style targeting Label in general, which affected the label control inside the Button I guess. Not super-expected behavior maybe?
Anyway, ignore the absolutelayout stuff :)