Hi Jan,
you are right, generally the term with the lowest p-value is selected as leading group term except for the cases in which groups overlap. In this case, we select the next most significant term. This is the reason for which you see highlighted in the network "ribonucleoside bisphosphate metabolic process". The term "organophosphate metabolic process" is part of four groups, and already highlighted for one of them (light green). Please switch between ClueGO visual styles if the overlap of the groups is not visible on the network.
You can highlight additional terms on the network in the ClueGO+CluePedia panel/ClueGOResults table (the third button from the left side, "Label", will update term names on the network).
Best