I used the quick-and-dirty focus curvature test (Photoshop > Filter > Stylize > Find Edges, then convert to B&W) on a picture of my driveway and entry pavers with the 30 mm f/3.5 Fuji G-mount lens wide open on a GFX 50S.
This is excellent performance. Compare that with a more typical wide lens.
Really impressive performance here. I can’t remember the last time I saw a wide angle lens with this little field curvature, or at least some mustache shaped curvature from higher order correction.