While searching mine shafts in the snapshot I found that spiders spawn inside the walls and then die. Also, I found that some even live after that and stay in the walls and cannot get out. Also, while dealing with this issue it seems that torches don't seem to affect the spawner. Meaning they still spawn even if there is light. I'm not sure if that is a bug or if they have always worked like that.

To recreate:
1. Find a mine shaft with a spider spawner surround by stone.
I'm not sure if it's able to be recreated by you. I think it has to be naturally generated so I will include the seed and xyz

I found this on my own server and recreated the world off the server and still got the same results

Seed: 8700268998187482873
X: 637.000
y: 26.000
z: -706.000