Preinfintiy and Postinfinity settings within the Curve Editor of an Animation can not be changed from Constant

This is a regression tested in //UE4/Release-4.27 CL 18319896

Preinfintiy and Postinfinity settings within the Curve Editor of an Animation can not be changed from Constant. Either by right clicking and changing the settings or choosing the settings from the drop down in the bar above the sequence.

1. Create a Third Person template project
2. Search for and open the Animation Sequence 'MM_Idle'
3. Select the drop down next to the Curves then select Add Curve > Create Curve
4. Name the Curve something then double click the curve to open the Curve Editor
5. Add 2 Key frames with different values for time and value
6. Right click the curve then select Postinfinity or Preinfinity then any option other than constant

Expect Results:
The curve updates to the selected setting.

Actual Results:
The curve doesn't change and the setting remains as Constant