underextrusion-like artifacts at small layer heights

vCore 3.1 nearly latest RatOS Rapido UHF (Volcano nozzle) I use variable layer height with OrcaSlicer 2, at the small layer heights (0.1mm) there is something like underextrusion or a partial clog (looks like it, but is not that). In the middle, where the layers are higher (0.2mm) the print looks fine. I swapped to a new nozzle (CHT clone) and it got a little better, but still visible (pictures are from before the nozzle change) I increased the temp from 210 to 220, this changed nothing for one PLA. For FormfuturaPLA higher temp improved it, but it also has higher recommended temps of 220-240deg. It first occurs in round perimeters, but if its bad (temp too low on FormFutura) also on straights. Klipper Autotune (tunes the stepper driver parameters) improved it significantly. So this is something like VFAs? Slower printing of external perimeters also improved it.