Unfortunately, it is impossible to make a round bevel on polylines in Houdini for reasons unknown to me, but this is a conscious choice of the developers, as they talked about. Bevel in Houdini has significant bugs, plus this strange solution for bevel of lines. You can get a round bevel on splines if your line is correctly converted to Bezier (and here you will have to add 2 additional points for each point) and then in the curve node you can manually make a round corner. But there are many nuances. A round corner will be made between straight lines. And if you connect a straight line and an arc, then for an accurate transition you need to ensure that the tangents coincide at the transition point. This can be done in a very non-trivial way - you need to make the last segment of the arc adjacent to the corner straight (we remember that it is Bezier), manually align it as a continuation of the penultimate segment of the arc so that their tangents are the same, for this you will have to shift the corner itself. We make the angle round, the tangent of the bevel adjoining will be equal to the tangent of the straight line of the last segment and coincides with the tangent of the penultimate segment. Accordingly, if we increase the angle so that it "eats" the entire last segment, then it will reach the point of the penultimate segment of the arc and their tangents will coincide. And we will need to connect the points. Voila, the perfect transition. But it is so difficult that it is easier to outline a cylinder, right? Or convert it to polylines, squeeze polygons out of lines, make a bevel on the corner (on polygons it will make a round bevel), select the edge and convert it to a line, think something bad about the one who came up with it.
Ahahaha, that's the whole Houdini - not to make a simple normal bevel, but to make incredibly twisted paths for the solution, which any roller coaster would envy.
If you do that with nurbs/bezier curves to start with, it's much simpler and much less code.
I do that with my modeling - the 'secret' being keeping all your prelimaray work done in nurbs/bezier space first, only converting to polygons when you are at the end of your building.
I'll accept that it might not be the best way to do it, but it works. If I wrapped it up in a blackboxed HDA you wouldn't be any the wiser into knowing how it's set up. Feel free to improve/redesign the technique~