Educated guess: it appears that currently, only the time reduction from the Thrusters upgrade is taken into account by the server, while reduction from both Thrusters and the trophy upgrade are used by the client. The jump in seconds that you are seeing comes from the server (incorrectly) correcting the client-side timer.
With Thrusters level 10 (10%) and Expedition Reduction level 11 (18.5%), my actual reduction is almost exactly 10%. The 'almost' part is most likely due to my measuring of actual duration being slightly off because pressing buttons on stopwatches is hard.
With Thrusters level 10 (10%) and Expedition Reduction level 11 (18.5%), my actual reduction is almost exactly 10%. The 'almost' part is most likely due to my measuring of actual duration being slightly off because pressing buttons on stopwatches is hard.