By default, DesignLife assumes a 2D stress state for the surface nodes. The embedded Add-On uses that default assumption. You cannot change that behavior in the current release of the Add-On.
If you use standalone or integrated Ansys nCode DesignLife interfaces, you can force DesignLife to evaluate a 3D stress state. You need to change the "SurfaceNodesOnly" and "ResolveToLocal" settings to "False". Those options will be included in a future release of the Add-On.
Be aware that with those settings, the critical plane calculations will be much more extensive. You probably want to use the "SelectonGroupType" to limit the analysis to a subset of nodes.