Singlemode is designed to minimized modal dispersion but it still occurs, especially over the longer distances including from stresses in the core rather than traditional bouncing. First photon isn't as important, it's the signal peak that matters as the receiver will try to decode from that. Typically this latency is hidden by needing repeaters every ~60km anyways due to both dispersion and loss. I'm not sure how far apart Starlink inter-satellite can repeat and if they can "skip" satellites as long as there is a clear direct path to another farther along the path, I haven't been in contact with their engineering folks since I changed jobs last year and am no longer a corporate customer.
