Sometimes, when rendering, shadowy shapes may appear in areas of a garment where the fabric is more bunched together, such as under the arms. This phantom shadow effect is typically related to HDRI lighting issues that occur where certain angles lack proper light definition.
The following image shows an example of phantom shadows.
If this occurs, try the following fixes:
Use a more ambient light: one which is less directional. For example, it is recommended to use Studio Soft lighting.
Either reduce the depth of the normal maps, or use normal maps with a more subtle effect.
Rotate the angle of the HDRI lighting to one that has more ambient light.
Use a smaller grid.