This is the B450 Pro4 second revision. I took a pics from a different angle. It still bothers me tbh.You're fine! Motherboard PCB's tend to have a slight warp at the middle when you don't have a thick PCB layer. Just to corroborate that point, what is the model for your ASRock branded motherboard? PCB's are also flexible, meaning when you install the motherboard in your chassis or you add weight to the board with the cooler and discrete GPU when you breadboard, you'll see the board is flattened a little bit more.
As for returning it, if the board doesn't POST with the bare minimum(while breadboarded) then yes you return it.
I'm worried that it might cause micro-cracks in the future, as the board looks quite thin.Not to worry.
When mounted in a case, the motherboard screws will eliminate any bend.
It looks very slight to me and I would not return it unless it proves defective.
The copper traces are very flexible; I would have no problem installing it and using it. But if it's a concern you find intolerable then go back to your retailer and try to return it; hopefully you used one with a good return policy. I tend to doubt the mfr. would RMA the board unless and until it doesn't operate (what is Asrock's warranty period for the board?). As @Lutfij said: install it to see if it doesn't work first before going down that path.I'm worried that it might cause micro-cracks in the future, as the board looks quite thin.