I have a survival world that started in 1.14, is run as a small server and has been updated regularly. Since the last small update (1.15.2) We have been experiencing thunderstorms every single day just before midday in-game. When this happens the light level drops so dramatically that monsters will spawn all around the player.
This entire process can take 30 seconds to go from full daylight to dark enough for monsters. Every single in game day we have to sleep through the storm.
The biome only seems to affect the rainfall, and not the actual storm nor darkening to night-time levels.
If a player waits (or sleeps) through the storm, it will be early afternoon in-game by the time the sky is clear and the bed will need to be used to skip night within a few minutes every day of this thunderstorm.