A line across only the first few frames is most commonly a scratch from debris somewhere in the film path, or damage introduced during processing. Based on the answers, the likely causes are:

• dirt/grit inside the camera scratching the film as it advances
• debris left behind during or after the CLA
• something in the developing reel/tank/process scratching the film during loading or drying

Because it affects only the beginning of the roll, a small piece of debris may have scratched the film for a few frames and then moved or fell away.

What to do:

• inspect and clean the camera’s film rails, gate, pressure plate, and take-up area
• check your developing reel/tank and handling process for anything that could mark wet film
• run another test roll to see whether the problem repeats

If the next roll is clean, it was likely a one-off piece of debris or processing issue. If it happens again, the camera should be inspected again, especially since it was recently serviced.