This happens quite a lot... Something in the model changes (externally or internally), and that forces the extrusion process to shift in some fashion. If you have the start location optimized for speed, the starting point will move around as the model changes, and the "seam" where it star...
Looking for a bit of help. I have never seen this behavior before, please see the picture. At the thinnest part of the print ( due to the hole ) this print the layer characteristics change. ( 100% infill ) Note, this is CarbonX carbon fiber "infused" Ultem on a high temp printer ( 385 degr...