Under Extrusion on Top Layer Corners

[DbBRICK]

I have been chasing this issue for a while now and have tried virtually every setting. Essentially, no matter the number of top layers, infill, temperature, extrusion multiplier, print speed, acceleration, and jerk settings, I cannot get rid of this under extrusion. It happens with every material and every model with a flat solid top surface. Everything else about the prints is perfect so I am stumped on what could be causing this. The under extrusion appears to only show up when the print head is changing speed more frequently as the under extrusion does not show up on long travels.

The photo above was printed on an FFCP with:
PLA at 200C
6 top layers
1.11 extrusion multiplier (my calibrated multiplier is 0.99)
3600mm/min print speed with an 80% reduction for solid infill
0.2 layer height, 0.4 path width with a 0.4 nozzle
30% rectilinear infill

[S3D-Alex]

I know you said that speed and acceleration/jerk have no effect on this but it does look like the extruder is unable to keep up when the nozzle is actually moving at full speed. As it slows to go into the corners and double back the extrusions look solid and full again. This usually indicates slowing your print speed would be your best option.