Yes, this is normal. Saturation point is lower in cold water; increases with temperature.
Similar to sugar, easily dissolved in hot water, but takes time in cold water.

Dissolving borax needed a high enough ratio of hot water to powder to dissolve almost completely and the solution must be stirred for about 20-30 seconds to get going. i use borax to remove molds from windows or bathroom walls, even in clothes with molds .. it works! plus a dehumidifier, surely molds wont go back.