Perhaps your planet is surrounded by a thick dust field. The dust field is thick and surrounds the planet, and is approximately 5km thick.

The rain is still drinkable with minimal effort even if bitter.

The dust is just on the edge of orbit but far enough away that it wouldn't ever descend onto the planet and cause problems with the environment.

I'm not an expert but I would imagine that this would block a fair amount of light from entering the atmosphere, whilst still providing adequate enough sunlight to allow for normal life cycles to exist.

But why is there a dust field?

The space around your planet is or was a minefield, asteroids are or in the past (few thousand years perhaps) colliding with each other and the remaining fragments ricocheted off into deep space or down onto your planet (This could open up for some extra story telling, my thinking would be either an explanation for a particular species of mushroom or an event that the locals celebrate annually).

Now all that is left are small rocks and a lot of dust just orbiting the planet indefinitely.

Perhaps your planet is surrounded by a thick dust field. The dust field is thick and surrounds the planet, and is approximately 5km thick.

The rain is still drinkable with minimal effort even if bitter.

The dust is just on the edge of orbit but far enough away that it wouldn't ever descend onto the planet and cause problems with the environment.

I'm not an expert but I would imagine that this would block a fair amount of light from entering the atmosphere, whilst still providing adequate enough sunlight to allow for normal life cycles to exist.

But why is there a dust field?

The space around your planet is or was a minefield, asteroids are or in the past (few thousand years perhaps) colliding with each other and the remaining fragments ricocheted off into deep space or down onto your planet (This could open up for some extra story telling, my thinking would be either an explanation for a particular species of mushroom or an event that the locals celebrate annually).

Now all that is left are small rocks and a lot of dust just orbiting the planet indefinitely.

Edit

To better answer your question, if you did want to add a little bit of magic, perhaps there is a spell/magic barrier that prevents the dust from entering the atmosphere. I feel that this would better explain why the dust is stuck orbiting the planet rather than being just on the edge of orbit, as that doesn't sound particularly scientific.