This is likely the problem. ART has a concept of model pose, and rig pose. The ideal model pose usually is not a T-pose, but the ideal rig pose (at least on bipedals) is a t-pose. When you go to the deformation phase, a box pops up asking you to create a rig pose, and there is even an option to auto-create one to a t-pose. If you just hit save pose without doing anything, your rig pose is now the same as your model pose, which is fine, but will likely cause the issues you are seeing. If you were to go back to your export file and go back to placement, then under the “reference pose manager” menu, choose reset rig pose. That little prompt will come up again. This time, try choosing the Auto-align option and then rebuild the rig. That will likely fix the issue (if matching is a large concern).