Mobs still spawn from spawner in high light level

What I expected to happen was when I light up a dungeon, mobs stop spawning.
What happened is mobs still spawn even though the whole spawner is surrounded by torches. A big part of the mobs spawn really glithy inside chests and walls aswell, then they die of suffocation.

I noticed this before with a spider spawner last week, I did again today with a skeleton spawner. I am new to the bug reporting thing, I didn't think anything of it before. Could not find anything on the matter, figured I would post it.

I do not know how to recreate this, it justs happens to me in several worlds. I can record it if needed 🙂