When reporting a bug/issue:

• Screenshot
  Teardrop.sfd's Q:
  
  When roundtripped:
  

  Teardrop.sfd's R:
  
  When roundtripped:
  

  Teardrop.sfd's S:
  
  When roundtripped:
  

• The FontForge version and the operating system you're using
  21d929b, Win32 (also tested same version on Xubuntu)

• The behavior you expect to see, and the actual behavior
  FontForge generates a bad UFO that neither it, nor fontmake, can understand. As @skef has pointed out in Optionally only interpolate compatible glyphs/layers #4321:

  01:19:22 skef As I note in that bug it's just a FontForge design error that you can create a cubic spline with one control point on a segment.
  01:20:07 skef Any time there is one control point the other control point should be treated as present and co-located with its on-curve point

  Our S.glif gets written, though, as:

  <?xml version="1.0" encoding="UTF-8"?>
  <glyph name="S" format="2">
    <advance width="1000"/>
    <unicode hex="0053"/>
    <outline>
      <contour>
        <point x="123" y="524" type="line"/>
        <point x="609" y="716"/>
        <point x="823" y="524" type="curve"/>
        <point x="823" y="208" type="line"/>
        <point x="123" y="208" type="line"/>
      </contour>
    </outline>
  </glyph>

• Steps to reproduce the behavior

1. File teardrop.sfd
2. Export UFO
3. Reopen UFO, or use in fontmake

• (optional) Possible solution/fix/workaround
  Always write two off-curve points for cubic splines. Co-locate one if missing.

Can you please fix this @frank-trampe ? I don't have time to patch it right now.