plan UI "copy time & instruments from previous" gets overwritten when adding time with buttons

In the plan UI, I adjust the time for the first waypoint, open waypoint 2 and copy this time. Seems to work, but when I add +1 day/hour/30 min it adds this time to the original time of the waypoint, not to the time just copied from the previous. 

I expected the time that was just copied to be adjustable with the +/- time buttons. 
