If an event is clearly visible, all market participants are aware of it, it has mostly no impact. It is priced in before. Or, the contrary happens: All are buying before, and selling after the event.
The halving reduces supply but the demand is variable. Millions of bitcoins are intact, not touched for years, on some whale's accounts. That could be a far more important supply than the effect of halving.
Although, the long term effect of halving can be positive, of course.