Yeah, thanks, I considered the possibility of a preset not correctly centered.
However using an APS-C lens full open @f/2.0, and with a 1.6x crop preset, I guess that vignetting should come up to be huge (literally a black corner) in that case. Instead, vignetting was heavy (partially compensated in post), but in line with expectations (given the configuration and the use of two stacked medium-quality ND filters).
Also, I thought that using the 3.5K 1:1 centered preset, the frame cannot be moved. But I'm not sure. Will take a look.