Hostile mob spawner wont stop spawning mobs?

Hostile mob spawners keep spawning mobs when they have enough light. 

I put torches all around the spawner (even tried with glowstone), but the spawner keeps spawning mobs.

I have tried it on 2 different worlds where i found a naturally generated spawner (zombie and spider). 

The problem is also on 1.16.4, havent tried anything below.