Hey. There was an update (1.9.5.0) that reset the Japan discovery flight allowing you to get progress towards Decathlon a second time. The patch notes say multiplayer was removed for the discovery flight so maybe that tinkering had something to do with it counting again after the patch.
I know for certain I got progress towards Decathlon after flying the Japan discovery tour (added in 1.9.3.0) and another 10% progress after flying it again post 1.9.5.0.
So while there have been only 8 activities, 90% progress is possible if you flew the Japan discovery flight after 1.9.3.0 and again after 1.9.5.0