>At what altitude? Earth isn't flat, so even at 20km and superb electronics you don't see what happens near the ground.
Over distances of 600km the vertical offset due to earth's curvature is about 20km. If it flies at it's operational ceiling of 14km, it can offset most of that but wouldn't have direct line of sight on planes at extreme range flying below 6km. If it has over the horizon capability though, which honestly seems highly likely for a recently updated system, then it should be able to compensate for the remaining shadow with some loss of resolution. Which I'm guessing is why it's estimated range is considered 600km. I'm sure the military analysts making those estimates know about these effects.
Anyway, southern Ukraine can be directly radar imaged from over Crimea so it's really not an issue. It's petty clear Russia can have full radar coverage of all of Ukraine without having to put their air radar assets close to Ukraine.
It is of course possible Ukraine might decide to try a long range anti-air strike, but I think that's unlikely due to the possibility of hitting a civilian plane, or the target coming down on a civilian area.