I just did a test with 15 x 15 x 3 mm cubes spaced out over the print area. I wanted to check if there was a large variation in the results as I've seen some lateral variations in previous prints. This test was done with PLA/PHA at 230C, 0.2 mm layers, 50 mm/s and 0.6 mm top/bottom surface. The results did vary a bit but on almost all places the top layer is not covering the surface and the side walls.
Judging from what I've read and seen it looks a lot like what seems to be coupled with the short belts not being tight enough. A couple of days ago, I did unscrew the motors and push them as far down as I could so I'm not sure if that is what really going on or if there is anything else that can be done.
Considering that the extrusion cylinder test looks fine on this instrument, could it still be under extrusion somehow? It really does look like it's not feeding enough material although this print was done at 4 mm^3/s (which works fine in the extrusion test at 230C).