Not a bug. Choose Marion first, if you want her at all. Yveen wants to pair Sandy and Random, I think... There's even a hint in the memories Yveen gives to Wendala that Random and Sandy are sweet on each other.
In fact one the second Phoenix heart is always used on Alexandra or Sandy. I'll make examples:
1) You revive Sandy. Next Wendala will choose Alexandra. 2) You revive Alexandra. Wendala will choose Sandy. 3) Revive Marion. Next you will have the choice between Alexandra and Sandy.
So the only chance how to revive someone else then Alexandra and Sandy is to choose the person first.
Here is the full tree of choices. Basically, you will get one of Alexandra / Sandy. You can choose Marion / Leona, but only if you choose them first. You cannot get both Marion and Leona.