i have put it up before and so has my son, we have both lost big fleets because of this bug! today it happened again but i have seen it in time and i can send vespene to fix. i send this private as i do not wish to reveal this planet to everyone, i have a planet at [xxx:yyy:3] i sent out all my fleet for a attack from coordinates [695:713:3] (one fleet to attack) (one fleet to recycle) (one fleet to occupy and destroy fleet on coords) however the third feet says that it was sent fom my planet [xxx:yyy:3] and has not enough vespene to return this is wrong it was sent from cords it should return to cords not planet    (fortunately  on this occasion i saw it and was still around to send vespene so i will be able to save this fleet but it is still wrong)

do not say this is not a bug it is a bug the ship should return to were they were sent from

If you send units without extra vesp to occupy/defend a coordinate they bring enough fuel for the selected amount of time and ONLY that amount of time.
If you send additional units to that coordinate on the same mission type the fleets will combine into 1 fleet.

This will extend the duration the the initial ships arrived stay as they join up with fleet 2. 
This can cause them to extend beyond their originally ordered amount of time.
By staying longer they have consumed too much fuel to return home at regular speed (you can click sending, then reduce the travel speed)
You either need to ship a little extra vesp with them, send a little extra vesp with the 2nd fleet movement arriving there, etc.

I am not a mod, but this isn't a bug. 

Practice using these functions near your planet where you are not at risk on either end so you can easily pull them back home without being caught.
Otherwise you seriously risk losing your ships to this mechanic.

Plz note the main rules of flight at coordinates:

Fleets that are bound to the planet and sent from coordinates with the missions "attack" and "recycling" will return to the coordinates.

Fleet sent from coordinates with mission "defense" to other coordinates or with mission “occupation” will return to the planet (object) to which it is bound.

Binding to the object is obligatory when you leave the object. Binding to the object can be removed after the teleportation of the fleet through the gate of teleportation.

When fleets are merged at coordinates, the fleet will return to the object fleet from which arrives at the coordinates first.

It is not a bug.