ID 438596 - When the distance between two keyframes of a spline point is too small, the interpolation of the curve changes to Constant

Problem summary
When the distance between two keyframes of a spline point is too small, the interpolation of the curve changes to Constant.

This issue can be reproduced with any node that uses splines, such as Roto nodes or SplineWarp. This issue does not seem to occur for curves when animating knob values.

Customer reported version
Nuke 12.1v2

Customer reported platform
CentOS 7

Steps to reproduce

1) Create a SplineWarp node.

2) Select the Pin Tool and add a point somewhere in the Viewer.

3) Go to another frame and animate either of the points by a few pixels. 

4) Select the points in the Viewer > Right click > Curve Editor > Points

5) Select any one of the curves, for example Pin1 > curve > 0 > main.x

6) Frame all keyframes (F key in the Curve Editor window) and drag one of the points up or down to bring it closer to the other point.

7) Notice that the curve will switch to Constant interpolation when the values are too close. This seems to consistently happen when the difference between the two points is around 0.05 or less.

The same steps can also be reproduced with the same steps when using the spline tools, instead of the Pin Tool.

Expected behaviour
The interpolation of the curve should remain the same.

Actual behaviour
The curve will switch to Constant interpolation when the values are too close.

Workaround
Unknown.

Reproduced by support

This bug has been reproduced in:
Nuke 12.1v3 - Windows 10 - macOS 10.13 (High Sierra) - Linux CentOS 7

Nuke 12.0v1 - Windows 10

Nuke 11.0v1 - Windows 10

Nuke 10.0v1 - Windows 10

Nuke 9.0v1 - Windows 10

Nuke 8.0v7 - Windows 10

Nuke 8.0v6 - Windows 10

Nuke 8.0v5 - Windows 10

Nuke 8.0v4 - Windows 10 - macOS 10.13 (High Sierra) - Regression

Unable to reproduce bug in:
Nuke 8.0v3 - Windows 10 - macOS 10.13 (High Sierra)

Earliest version tested
Nuke 8.0v3
- This issue doesn't appear in this version and has regressed