Could be evidence of “gap infill” transition with classic wall, arachne changes it to innerwall. See pics. First pair are Classic. Second are Arachne.
The transition maybe cause the step due to either widths or speeds of the gapinfil/innerwall change.
With archane its all inner wall through the transition.
I’ve noticed this with a model of mine with a thinnish cylinder wall that had a random change and chased it down to this.