System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.07

Blender Version
Broken: version: 3.0.0, branch: master, commit date: 2021-12-02 18:35, hash: rBf1cca3055776
Worked: (newest version of Blender that worked as expected)

Short description of error
When using the RGB/Vector/Float curves nodes and choosing the Extend Horizontal option, the output is the same as with Extend Extrapolated if the X value of the last point of the curve widget curve is greater than 1 and the Y value is smaller than 1.
The inverse is the case for the first point, so the output is the same as with Extend Extrapolated if the x value of the first point is smaller than 0 and the y value is greater than 0.

The image below shows the areas where, if you place the point, the result in the render will show as Extrapolated instead of Horizontal:

Exact steps for others to reproduce the error

• Open attached file
• Set Cycles Render Preview display mode to better visualize the problem through displace (Eevee has no such issue.)

The test files only include the case for the last point and for the RGB curves node, but everything is applicable to the first point and the Vector/Float curves node.

Each of the three objects shows the wrong result. The beginning and end of the geometry should be Horizontal.