Ender 3 build surface is sticking up in the middle
So your (clone) BuildTak bed surface is not sticking to the bed anymore, time to remove the surface and clean the heatbed and apply a new one. These build surfaces (usually) have a sticky 3M sticker to stick it to the bed (or the surface bottom is covered with a sticky surface). When this sticking layer fails of subsequent heating cycles you need to replace the build surface. The higher the temperature the more the build surface expands the higher the surface gets when it doesn't stick in certain places. 

Note that the build surface is a consumable, it needs to be replaced once in a while.