It's a problem with the coating.
The phone is originally designed so that the slits are coated.
I can only minutely reproduce the issue on my phone regarding signal. I have no wifi problem.
The coating was improperly or applied too speedily to the phones. If it were really a design issue, I can touch the right side and it would drop signal also. And the top for that matter.
The coating is to prevent conductive contact with flesh and the antenna.