the tattoo ?????? how did it come to an ending without them getting a bonding tattoo !?

Cause they’re both alphas

The author answered this in the creator's note ch.

Q: Will the two ever have the same mark on their bodies?

A: If Colton and Jinwoo make an effort(?) the locations of their tattoos could change. The scar shapes won't change since they're alphas, but they'll change to something only they can recognize.