I call these “pencil lines” and I get them on various 3D carves. On one specific model they happen in a predictable pattern based on the cut strategy. I have been battling it with various upgraded and approaches. But from what I can tell it is a couple of different forces at work. But the simplest source is “flex”
Material flex. bit flex, machine flex.
Some materials are more prone to it. Which leads me it is in part a interaction of bit and wood. (Pine is the worse, while the denser woods carved much better - I think in part because of the slower cut speeds?)
Some cut strategies are more prone to it. Again angle of bit to wood, cutting across gran vs with grain, etc.
First: Gantry flex. With the old stock gantry it was a big issue. The gantry would flex under load, causing the bit to deflect to angle, changing the depth of cut just enough to show up as a line.
I did a bar mod that helped a lot. Later I replace the gantry with c-beam. This almost eliminated it completely. I hear that the upgraded gantry also eliminates most of the gantry flex and well worth doing if you have not already.
Second: Material flex. Larger spans of material can lift up while cutting. Especially when you cut it down to a very thin thickness. I switched to a downcut bit for my rough cuts and try to leave a thicker base of material on my designs.
Third: Bit Defection. The bit can deflect under load, but to be honest when using shorter bits I have not seen it. The gantry and rails deflecting long before the bit does. I have see it with longer bits, so I stopped using these except where necessary. For 3D carving I switched to a tapered bit. This worked really well. The tapered bit gives you the benefit of the small tip for fine detail but a thicker shaft for less defection / breakage issues.
And finally: everything else. As always check you wheels, your belts your pulleys. I had issues just last week. It turns out I had a squished belt tooth and a loose pulley.