setStartSegment puts off curves at the beginning of contours.

[typesupply]

This is undesired behavior because somewhere in the variable font tool chain an on curve is expected. Here's a demo of the problem:

from fontTools.ufoLib.glifLib import readGlyphFromString

def dump(glyph):
    for contour in glyph:
        print(contour.getIdentifier())
        for i, segment in enumerate(contour.segments):
            print("+", i, segment.onCurve.getIdentifier())
        for i, point in enumerate(contour.points):
            print("-", (i, point.getIdentifier(), point.x, point.y, point.type))

glif = """
<?xml version='1.0' encoding='UTF-8'?>
<glyph name="A" format="2">
  <advance width="500"/>
  <unicode hex="0041"/>
  <outline>
    <contour identifier="44XxVKnB2x">
      <point x="500" y="250" type="curve" smooth="yes" identifier="sAwqsKyYNW"/>
      <point x="500" y="388" identifier="6tQSHOf9Ys"/>
      <point x="388" y="500" identifier="0RDMteuRyw"/>
      <point x="250" y="500" type="curve" smooth="yes" identifier="Dqlm7jnjuf"/>
      <point x="112" y="500" identifier="VdNjzcUQ07"/>
      <point x="0" y="388" identifier="cIkDdREiYg"/>
      <point x="0" y="250" type="curve" smooth="yes" identifier="6eCPXmIBoJ"/>
      <point x="0" y="112" identifier="DnlDWVnzCn"/>
      <point x="112" y="0" identifier="othuknxJCG"/>
      <point x="250" y="0" type="curve" smooth="yes" identifier="kCNyF3Qqen"/>
      <point x="388" y="0" identifier="HfzlQB6wyA"/>
      <point x="500" y="112" identifier="uVNo2cq7Pf"/>
    </contour>
  </outline>
</glyph>
""".strip()

glyph = RGlyph()
pen = glyph.getPen()
readGlyphFromString(glif, glyphObject=glyph, pointPen=glyph.getPointPen())

print("Original:")
dump(glyph)
print("")

contour = glyph.contours[0]
contour.setStartSegment(1)
print("Changed:")
dump(glyph)

Original:
44XxVKnB2x
+ 0 Dqlm7jnjuf
+ 1 6eCPXmIBoJ
+ 2 kCNyF3Qqen
+ 3 sAwqsKyYNW
- (0, 'sAwqsKyYNW', 500, 250, 'curve')
- (1, '6tQSHOf9Ys', 500, 388, 'offcurve')
- (2, '0RDMteuRyw', 388, 500, 'offcurve')
- (3, 'Dqlm7jnjuf', 250, 500, 'curve')
- (4, 'VdNjzcUQ07', 112, 500, 'offcurve')
- (5, 'cIkDdREiYg', 0, 388, 'offcurve')
- (6, '6eCPXmIBoJ', 0, 250, 'curve')
- (7, 'DnlDWVnzCn', 0, 112, 'offcurve')
- (8, 'othuknxJCG', 112, 0, 'offcurve')
- (9, 'kCNyF3Qqen', 250, 0, 'curve')
- (10, 'HfzlQB6wyA', 388, 0, 'offcurve')
- (11, 'uVNo2cq7Pf', 500, 112, 'offcurve')

Changed:
44XxVKnB2x
+ 0 6eCPXmIBoJ
+ 1 kCNyF3Qqen
+ 2 sAwqsKyYNW
+ 3 Dqlm7jnjuf
- (0, '6tQSHOf9Ys', 500, 388, 'offcurve')
- (1, '0RDMteuRyw', 388, 500, 'offcurve')
- (2, 'Dqlm7jnjuf', 250, 500, 'curve')
- (3, 'VdNjzcUQ07', 112, 500, 'offcurve')
- (4, 'cIkDdREiYg', 0, 388, 'offcurve')
- (5, '6eCPXmIBoJ', 0, 250, 'curve')
- (6, 'DnlDWVnzCn', 0, 112, 'offcurve')
- (7, 'othuknxJCG', 112, 0, 'offcurve')
- (8, 'kCNyF3Qqen', 250, 0, 'curve')
- (9, 'HfzlQB6wyA', 388, 0, 'offcurve')
- (10, 'uVNo2cq7Pf', 500, 112, 'offcurve')
- (11, 'sAwqsKyYNW', 500, 250, 'curve')