When inside the TIMELINE node. If you add a color channel and set its value to anything more than 1, after closing the color picker dialog it resets to 1!
Typically colors in UE go from 0 to 1, not from 0 to 255. So 1 is the max value.
If you want to use the curve for anything else than just a color, maybe you should use another curve type.
Closing the ticket. this was fixed in 4.22