What makes you sure that attacks in Belgorod aren't the same as the 'terrorism' that prompted the resumption of the Chechen war? In that case Putin killed quite a few innocent Russians (including children) in order to have a causus beli. Sound familiar?
Up to now, the Ukranians have been careful to only attack military targets. As you mention, some of their support is getting less staunch. I can't imagine they don't know this - and understand how an attack on civilians would further that process. Further, I think they understand how little Putin cares for civilian lives and have no expectation that killing some might get the Rus to, say, stop launching terror attacks on Ukranian civilians.
Color me unconvinced.