I'm struggling to understand how to farm grineer kuva lichs. as far as I understand you can go into a grineer mission above level 20 and if a kuva lich spawns you can kill it and it'll drop a kuva weapon?

I've been running to capture mission Cassini and I do get lichs when i get lucky except when I kill them theirs no weapon? I've completed the call of tempestarii as I read somewhere that is a prerequisite. but still no luck in the lich dropping a weapon I do get 100 Luca each time though. I'm pretty sure you can get kuva weapons with this method though as people who I have ran the mission with have told me they've recieved one.

is there anything I'm missing? I also read on a couple websites that you can get kuva weapons this way but in struggling to understand how. any ideas or explanations will be gratefully recieved.