I'm using an Ender 3 Pro with stock heated bed & glass build surface on it (without any base of stock magnetic plate). My heated bed looks completely flat too despite of many uneven Ender 3 heated beds around.
But when bed temperature set to 70 °C and I check the temperature of bed with infrared laser thermometer gun, the front of the bed is around 72 °C and the back is around 64-68 °C. So the temperature is different at all sides and this looks like leading to warps too.
Is there any way to make it more even for all bed surface?