Does the Bend command always produce a developable surface?
No.
In general Bend only results in a developable surface if the original surface is planar, or if the original surface has only single curvature and that curvature is in the same plane as the bending. There may be a few other special cases.
Use DevLoft or DevSrf to create arbitrary developable surfaces.
https://docs.mcneel.com/rhino/7/help/en-us/index.htm#commands/devloft.htm