I would presume it has something to do with the fact that when you 'section' you aren't really generating an entity (or surface). Thus, you may not end up where an exact node is (even if you could select it at that point).
I agree with James; the 'section clipping' is just for graphics and doesn't create a selectable entity.
The workaround is to create a face where you need it prior to running FEA. Use a plane to split the body, for example. In the simulation make sure the two bodies have a bonded condition so they behave as if they were not split. Then you can probe the results plot on the desired face.
This can also be useful for viewing results; you can create an exploded view and view your results on that.