Printer: SecKit Go w/ BMG/v6 and SKR v1.3/TMC2130 Drivers
Slicer: Simplify3d or Cura (problem is much worse in Cura?)
Long story short - after getting this printer running, I have noticed that it will underextrude ONLY on corners on an internal perimeter specifically once we're above bottom layers - and nowhere else. See below:
E-Steps have been calibrated, extrusion multiplier has been calibrated (single perimeters are 0.48 mm), linear advance has been calibrated, but here it gets weird. Anything resembling a 'standard' value (suggested between 1-2) results in MASSIVE underextrusion after the speed change. I was able to get a good looking line using the linear advance test with a value of 0.15 (which seems very low).
Retraction (1 mm @ 30 mm/s), junction deviation (0.02) and all other settings are off the factory Simplify3d profile. Turning linear advance off makes the problem slightly better but doesn't fix it, going to a value of around 1 results in huge underextrusion after any change in direction.
I am absolutely lost as to what to try next.