Can you tell what the initial positions of the bones are in the Windows 3D viewer? When you say “starting positions of bones” I assume that means when ignoring any animations in the model, and not displaying, for example, the first frame of the first animation? Note that the animation playing back correctly doesn’t necessarily mean the bone starting positions aren’t the same in that viewer.
This question seems to depend mostly on the model and perhaps the software… By all appearances, yes, the bones are starting in a non-tpose position. Without knowing how the model was made, I can’t tell you how the bones got there… maybe it’s the same in the FBX? Made it’s a bug in the FBX2glTF converter? Maybe a bug in your viewer? There’s a lot of missing information needed to debug this sorry.