figured it out, the fleet kep getting added a move to earth movement due a conditional refueling order and that was messing with the tanker refueling the ships
I don't know if this is a bug or if I'm doing something wrong, I previously managed to refuel a stranded fleet with these very ships, but this time the colony ship have been staying at 0% fuel for multiple 5day and 30 day intervals.
here's the fleet setup after a merge % refuel order: