Overworld mob spawners (spider, zombie, skeletons) that are currently in a lit condition will begin spawning mobs after a thunderstorm event. The lighting must be disabled and then re-enabled in order to stop mob spawning. Definitely a bug related to thunderstorms.

This is happening on a realm. It started a while ago and confirmed it is still an issue on 1.13.0.

Attaching screen captures found spawned and spawning mobs in a farm area after a thunderstorm event where both mob spawners were left in a LIT condition upon last use.