There is no means to choose blocks variants; it will be the base type every time (i.e., filling with "planks" means it will be oak, and one cannot choose birch, spruce, etc.).
MC-122052
New fill command cannot select block variants
Resolved
Invalid
0
If you want to use diferent variants, do /fill x1 y1 z1 x2 y2 z2 planks[variant=spruce].