Ah, I better understand the knob model now that you've described it. Well, inside circles add a bit of a problem in that they tend to "pull" the extrusion towards the center of the opening as the nozzle moves around it. On normal holes, this can make round holes come out undersized. On the knob, that slight pulling might have meant the new layer wasn't gripping the offset layer below enough and it pulled free. In other words, it's easier to build that overhang on a straight edge than it is on a circle. Another thought is that there's just a bit too much of a gap on the first layer. If there is, that can mean the 2nd layer can't print reliably and this can continue up through the model. Once one layer doesn't stick to the layer below, there's likely no way the layers above will ever recover.

I know what you mean about part dimensioning. The only design tool I've used is openSCAD. Anymore, I don't bother to print someone else's Thingiverse item unless they provide the openSCAD source for it, just in case I need to adjust something. I also typically avoid models where there's no photograph of a finished print. I found there's too many people out there who like to crank out designs, but they have no printer so the design is unproven. On knobs, then you have the issue of 1/4-inch vs 6mm shaft diameters. On push-on knobs, the two sizes aren't interchangeable.

Adding the single solid layer across an unsupported area where there will be a center hole is certainly a way to go. The Greg's Wade extruder body and large gear models I started with in my overhaul of the designs did that. I learned why the solid layers were there when I took the solid layers out of my earlier revisions. Even with support added in the slicer, the center hole just could not come out cleanly.

Picking up 3D printing as a hobby has been fascinating, but if I knew before that I'd be having to learn so much about nit details I think I would have passed on it.