Just watching GOF and when karkaroff is being tried for death eater crimes In the past, we get a flash back when Harry jumps Into the pensive, karkaroff tried to bargain his sentence with information and names snape as a death eater.
Dumbeldore jumps up and says something along the lines of
‘Not this again, I’ve said before that snape was a death eater but now he works for us, at considerable cost to his own safety’
So dumbledore has just told everyone that snape is a spy, so why would Voldemort have his back in his gang in the present? It makes no sense, Voldemort isn’t stupid.
I feel a bit sad now that there is a plot hole.