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.
The curve updates to the selected setting.
The curve doesn't change and the setting remains as Constant
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-163181 in the post.