According to TikTok user @4jmjcbitxh, the flat spots on ice cube trays are where you're meant to fill in the water, not splashing water everywhere by trying to fill in the holes.
In a video posted in May that just went viral, she holds up an ice tray and instead of moving it side-to-side, she lets the water flow on the flat area of the tray mold filling the holes evenly and without the excess water dripping everywhere.