Warm naphtha? Is it possible naphtha evaporated during those 3 hours? I would guess it did, and/or the naphtha pull cooled down and crystals started precipitating. 500g is a lot of mimosa = a lot of alkaloids, so if naphtha evaps or cools down, it will certainly start precipitating
No need to let it stand, mix for some minutes, let it stand for a couple more minutes, mix again, etc, then pull, dont let it stand.