Can anyone clarify what these lines are?

Maybe this?

You nay want to search this forum for
strange
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This one is a common issue for various BL printers.

Funnily enough, my nozzle was slightly out of alignment after checking it against this print. I printed before and after the alignments on a single layer, the second time round it had improved quite a lot!

Much appreciated for this link.

I’ve found that part of the problem can be the top surface pattern. As some suggested, it might be laying down some lines in one area, working in another area, then coming back to the first after it’s cooled. Look at the printing pattern of the top layer by inspecting where the nozzle is moving in the slicer. Playing around with patterns like aligned rectilinear instead of rectilinear (IIRC) can make a big difference.