Tested with:
- qwt 6.2.0 and 6.2.1
- Qt 5.14.0

The spline's shape appear to be dependent from the current axes' values (min-max). If the axes' values change, the spline changes its shape.

Unfortunately I can not upload csv files so I can not provide you with data (my data contains 200 points) but I can show you two screenshots where the problem becomes visible.

Spline1.jpg
x axis: 23.7304 / 23.7766
y axis: -1011.7266 / -875.0032

Spline2.jpg
x axis: 23.7335 / 23.7705
y axis: -1001.1308 / -891.7521

The data did not change between the two screenshots, only the axes' values were changed slightly.

I do not know if this is sufficient, if you tell me another way to upload the points data, I will do so.

Btw, I tried the same data and axes' values with Excel and here the spline does not change its shape.