I'' add my opinion here though I'm no expert on this kind of things.
Couple of main reasons why green leaves would drop
1. Shock
2. Over watering
3. Under watering
I think the issue is more likely poor plant transpiration and here's why.
Though It seems your watering schedule seems normal (not over watering), without air circulation, you restrict your plant to transpire H20 during photosynthesis which is critical for a plant.
Why air circulation is important? The importance of your fan (and the myth of making stem stronger bs) is to provide air circulation. But why air circulation is important?
When plant transpire they create a layer of 'thin air' around the leaves that is full of moisture. Think about it for a second. Even if your RH reading is normal, without air circulation, the air around the leaves has very high RH. This is a protective mechanism of the plant out in the wild to prevent rapid water loss.
If you dont move this thin layer of air around the leaves, come to a point that its preventing the plant to release the necessary amount of H2O and all your watering is just accumulating at your most mature leaves and could cause premature drop or really brittle leaf stalk.....just like over watering.
This is the reason why we need a fan inside our grow area and not because it would make your stem stronger.
But again, it's just my opinion.