Alfalfa seeds escaping the sprouter with rinse

I don't know what sort of seed sprouter you have there, so it's tough to give you solutions. Mine is a perforated lid type thing, and I've had the same challenge as you with tiny seeds. I just drain the rinse water through a small size, very fine strainer to catch them, then open the jar and tap any escapees back in. I don't set the jar upside down so seeds are on the strainer lid, either. I sit is so seeds fall back to the bottom of the jar. Once they have sprouted a bit, the little roots then prevent fall thoughs.