Well the weird thing is that when the first replacement was sent out, I didn't even see it in the store as it arrived cracked too and they complained to them. I don't do anything out of the ordinary with them and aren't hit off rocks etc.
That line does look strange. Since you've received replacements and they've "cracked" as well, it might be something you're doing. But it does look strange. Although, that picture may not be a crack. It could be a deep scratch IMO.
I think a crack would appear much darker in color since the material has split and the light would cast a shadow. This, to me, looks like a deep scratch rather than a crack. Again, IMO.
Process of elimination:
How do you to treat your clubs (roughly or gently)?
How do you transport your clubs?
Are there a lot of pebbles/rocks/sediment where you play or practice?
Do you lean on your clubs while waiting for your turn (like a cane)?
Does anyone else use your clubs or handle them (children/friends)?
Hope this helps.