I often times see myself wasting ground troops, because a backup fleet arriving at system with an ongoing invasion will automatically drop all their manpower into the ongoing fight.

Maybe it could be an idea to add some user control here, so that it is possible to not waste a lot of invsion power? 

Circumventing the invasion behavior is only possible through manipulation of fleet size and destinations, and timing of fleets.... So where you express your intentions not through a game system that is made for that, but through 3 corners and abuse of other systems. 

This feels wrong.

all manpower in an invasion will be spread across all the ships in orbit, that shouldn't waste your manpower

I cannot follow you. What do you mean?

As far as I understood it: The ship moves to the system and auto-adds its manpower to the ongoing invastion, that is correct yes?

Then you move the ship somewhere else, the invasion continues and your ship has no manpower. 

Or am I wrong here?

When you move your ships away while invading you loose that manpower but if there are ships with free manpower capacity around the system that finishes invasion then it gets returned to them

Yeah thats true but I guess you still can't go straight to the next system with your other drop ships and invade those 2 separetely because all your guys drop on the first one?

Well, I would expect the "add re-inforcements to invasion" functionality to be user-initiated, not automated.

This might be our best, clearest option for players. We'll look into what's feasible.

Thanks for the feedback :)

Can you also consider an option to manually choose how much manpower goes on freshly built ships? May be adding an extra slider into the ship design menu?