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.
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Looking at these screenshots, it seems that everything was updated correctly, but space that was part of the dungeon's walls.
This bug is a duplicate of MC-142134
Please search before creating a new report, as there is one already there.
Johnibur
Note that the zombies spawning in these conditions caused the server falling behind, and crash upon exceeding maximum tick, like described in MC-142782, but I don't know if it's related, although fixing the light like described above and killing those zombie fixed the server lag.