Are glass beds supposed to be slightly convex in the center?

My ender 3 aluminum bed is warped. The inner part of the bed is convexed, I can tell since when I print on a PEI sheet, I have to taper the edges of the bed with aluminum foil or else I can visibly tell the filament comes out very thin and doesnt stick to the bed when the hotend is near the corners, and the extruder motor starts skipping steps when the hotend is in the center.

I bought 2 glass beds so far from different sellers on Amazon to get more precise bed leveling, and so I won't have to guess how to taper the bed (since glass is hard and it would basically even out any warping of the bed).

Both of the glass beds are slightly concave in the center, since I keep seeing the same problem. I think it is very unlikely my x-axis rod is bent, since that beast is made of solid square metal. Is it supposed to be this way, or is something else going on?