Broken: present in 2.83.3, 2.91.0, and 3.0.0 alpha
Short description of error
Both open and closed curves exhibit a "sausage effect" when a bevel is applied. This is not visible when the curve is closed and 'ResolutionU' is below 4. The issue is somehow attenuated by applying large amounts of smoothing (twist_smooth).
Below is a flat and symmetric curve showing this issue.
Exact steps for others to reproduce the error
Delete default cube. Create a Bèzier Circle. Enter Edit Mode, select all vertices, subdivide once.
In Object Data Properties, select Shape->Twist Method-> Tangent, and Geometry->Bevel->Depth: 0.2.
Toggle the Active Spline->Cyclic->U, and change the Active Spline->Resolution U between 4-5.
Your smooth symmetric curve will turn into sausages!
The effect is slightly different depending on the type of handles.