I’m not sure that’s exactly strange, it goes down as you would expect from 5sec and continues down as the duration increases. If you feel it goes down to sharply at 1minute you could specifically train for 1 minute; but looking at your longer power/time you look to be a diesel power house 
and it’ll be harder to maintain it if you focus on 1 minute and to be honest I would prefer to go harder for longer like you 