When selecting the Add Selected Curve Asset button with a Float Track in a Timeline, nothing appears to happen. However, if you delete the Float Track and then either add another Float Track or add a Selected Curve Asset, then you will add the track you chose in addition to a track of the curve asset based on how many times you selected Add Selected Curve Asset before deleting the first Float Track. This does not happen when adding a Vector or Color Curve in the Timeline.
Tested in //UE4/Release-4.26 @ CL 15973114 and this did occur, so NOT marking as a Regression.
When pressing the Add Selected Curve Asset button when a Float Track exists in the timeline, nothing will happen, then after deleting the Float Track and adding another Floak Track or a Curve Asset, the timeline will be populated with multiple tracks depending on how many times you pressed the Selected Curve Asset button.
The Selected Curve will be added to the timeline when the button is pressed
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-133119 in the post.