It's normal.Clutch basket and trans main shaft spin when engine is running,it's just not in gear.But when it's cold the oil is thicker,when it warms up it thins out and then there's too much resistance for the wheel to spin.In other words,the thicker oil will spin the gears with no resistance,like the wheel off the ground.
Back in the old days when manual transmissions used 80/90 oil,a small car fired up on a subzero day would stall or jump if you let off the clutch before a little heat was in the trans due to the oil got so dam thick it was like it was in gear.