This happened inside a dungeon, the player starting spamming torches on the ground, the lightning updated correctly at first. Then they started mining the cobblestone composing the walls when the lightning system started to "ignore some torches". And the zombies started spamming from the spawner.
Not a client issue (relogging didn't help but updated the client lights accordingly to the server's values, and zombies still spawning). Placing blocks didn't help, moving torches neither, this has been fixed by placing the cobblestone back, sealing the dungeon, breaking again, and setting the torches again.
On the screenshot, you can see: the lightning issue from the client (this is not vanilla textures but this is irrelevant), also I joined a screenshot with lightning checked on a torch bock in F3 mode.