I agree it looks like a manufacturing defect. I would contact the retailer from you purchased the Arc Ultra or Sonos Sales if purchased directly. In fact it may be a good idea to contact Sonos Sales regardless of where the Arc Ultra was purchased.
In had a white Arc Ultra and didn’t notice anything like is shown in your picture. The wife thought the white was too distracting against our darker wood finish cabinet so I exchanged it for black.
Looks like a dozen or so holes have been punched fractionally larger than the rest (giving the impression of a black line rather than being a black line…)