I believe helicopters produce a distinctive flickering radar return due to rotor rotation that allows radar signal processing to pick them out relatively easily.
This is why i believe the Russian air cover are absent.
On the respect of topographic tho. Belgorod city is quite challenging in radar perspective against low fliers. This is coverage for a ground based radar, with mast height of 5 meters and 40 meters. I made a tweet about coverage. with assumption radar is located in Belgorod international