Strange results coming back from the lab

Strange results on this film when it came back from the lab: all along the film was this white "wave" on the bottom. I can't really explain it to myself, besides a lab manipulation accident. I thought at light leaks problems, but the camera has no problem (as I shot other films after this one without any signs of light leaks) and the film was a fresh Kodak Ektar…
As it was in Cornwall I couldn't go back and ask the lab what they did wrong… well, that's analogue!