This is related to the 4.9.0git version, potentially also the recent addition of the support for non-square icons. I use the default ("xfce4-panel-menu") square icon and the panel is configured in the deskbar mode with nrows=2. With these settings, incrementing or decrementing panel "row size" occasionally causes the icon to be stretched horizontally to the full panel width. This only happens when the label is placed under the icon.
I would say that this is fixed already in the meantime (can't reproduce).