Percent and stock counts mean nothing in terms of hitboxes and hurtboxes. Even if you’re at 200%, if no hitbox touches you, you don’t get hit. Therefore 2 grabs that overlap on the same first frame need to grab-break or both whiff, just like two smash moves, shines, or other moves clanking. Any other determiner (including port) is arbitrary in this.
Many moves in the game clank when they are frame and priority-wise equal. Grabs are no different. They are perfectly even if both opponents land a grab on the same frame.