I'm trying to draw a triangle with rounded corners of 5px radius. I tried doing this first with the Corners Path Effects, but it clearly isn't a corner of radius 5px, unless I'm misinterpreting what it's doing (see screenshot with Green triangle). When I draw a rectangle with 5px rounded corners, it perfectly lines up with the edges of a circle of radius 5px , so I assumed it'd do the same for the path effects too.

So, I tried using the Pen tool and tangential snapping to snap the edges of the triangle to the circles, but when I try to do Path Union or create it using the Shape Builder tool, there's an extra node where the straight line meets the curved circle outline. Using the Shape Builder tool's outline, I can see that the edge is not tangential to the circle's edge, even though they're lined up outside of that view.

Is this a bug? Is there an easier way to draw the triangle with 5px rounded corners?

LPE Corners is precise using "Geometric Bounding Box" under Prefs->Tools:

I set that in the Preference but it's still not lining up. Something's not right here. And how about the lining up of tangential snapping? It seems to line up but goes off when joining shapes.

Ok, so I found out why the path effect wasn't lining up the radii. The setting "Use knots distance instead radius" is the cause of that. But even then, I don't understand why the tangents aren't sticking when joining shapes.