Maybe you have to have all three magics in your inventory at the time of the last kill and if you don’t you are bugged?
I can also rule this out as being the issue. I went from one to the other, in the same game session and on the same character, to collect these. I didn’t think to check my inventory because I saw the collection notifications pop up. It is only now that I see that items didn’t actually generate for Fleshgrazer or the vet fleshreaver.