I made a default cylinder using blender and exported it into fbx file. When I looked at the content of the file after converting it into json, I found the field "PolygonVertexIndex" (under "Geometry"-"Objects") contains:"3, 1, 63, 61, 59, 57, 55, 53, 51, 49, 47, 45, 43, 41, 39, 37, 35, 33, 31, 29, 27, 25, 23, 21, 19, 17, 15, 13, 11, 9, 7" and " 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60".
These consistent positive indices series confuse me a lot since I believe fbx PolygonVertexIndex store indices in "positive, positive, negative" (when the surface is triangular) or "positive, positive, positive, negative" (when the surface is quadrilateral). And I cannot find any information regarding consistent positive indices when I googled.
The whole list looks like:
["PolygonVertexIndex", [[0, 1, 3, -3, 2, 3, 5, -5, 4, 5, 7, -7, 6, 7, 9, -9, 8, 9, 11, -11, 10, 11, 13, -13, 12, 13, 15, -15, 14, 15, 17, -17, 16, 17, 19, -19, 18, 19, 21, -21, 20, 21, 23, -23, 22, 23, 25, -25, 24, 25, 27, -27, 26, 27, 29, -29, 28, 29, 31, -31, 30, 31, 33, -33, 32, 33, 35, -35, 34, 35, 37, -37, 36, 37, 39, -39, 38, 39, 41, -41, 40, 41, 43, -43, 42, 43, 45, -45, 44, 45, 47, -47, 46, 47, 49, -49, 48, 49, 51, -51, 50, 51, 53, -53, 52, 53, 55, -55, 54, 55, 57, -57, 56, 57, 59, -59, 58, 59, 61, -61, 3, 1, 63, 61, 59, 57, 55, 53, 51, 49, 47, 45, 43, 41, 39, 37, 35, 33, 31, 29, 27, 25, 23, 21, 19, 17, 15, 13, 11, 9, 7, -6, 60, 61, 63, -63, 62, 63, 1, -1, 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, -63]], "i", []],
It can be seen that the indices are fine at the start of the list but goes weird later.
The list contains negative numbers to end a polygon and looks consistent. A polygon is not restricted to be a triangle or a quad, it can contain as many vertices as it needs.
One way to create a cylinder mesh is using quads on the sides and circles on both ends. In your case, the two long series of positive numbers are the two circles encoded as two big polygons containing 32 vertices each.