12 people were killed according to the governor of Kursk oblast'. Generally I'd be wary of believing anything that the Russian officials are saying but 1. it's hard to imagine no civilian deaths during such operation 2. this should be easily disprovable if it's a lie (it's not military losses that are classified) and I haven't seen anyone disproving it yet.
It's not first civilian deaths on the Russian side, people who live near the border are sometimes getting killed by both Ukrainian and (misfired) Russian rockets and bombs, so it's not something new. And yeah, it's hard not to look at it from a cynical Vardy-esque (chat shit get banged) viewpoint, especially after displaced people recorded a video message for Putin, saying something along the lines: "we've supported the special military operation from its very beginning, why did the war came to our homes?". But I try.