Take a new spreadsheat. Delete Sheet2 and Sheet2 to get smaller file. Switch of
size optimization for ODF format to get better readable file.

Enter some pairs of x|y values for example
x	y
0	-1
1	1
2	-1
3	1
4	-1
5	1

Generate an XY-Chart with smooth lines&points using B-Spline with "Resolution"=5
and "Data points order"=2.

Notice, that the curve goes approimately through (3|0.5)

Save the file.

Reload the file and double click the chart to actualize the preview. Notice that
the curve now goes through (3|0.2). Do not save.

Unpack the file and look into the content.xml of the chart object. The attribute
chart:spline-order is missing in <style:chart-properties>

If you use another value for "data points order", then the attribute is saved as
expected.