When using solid welds in the preferences of the welding tool (see fig. preferences.png) it is often not possible to create solid welds, because the material side of the weld is interpreted wrong by Creo (see fig. material_side.png).
Please add the (grey) material arrow for solid welds, too (compare material arrow from surface welds). When creating solid welds, it is not intuitive at all to use the button "Flip material side" without any feedback from the program.