Thank you, Andy, for your reply and advices.
Reimport and trying to relink the footage did not solve the problem.
But I found the mysterious icons are linked to a “non-logical” connection in the Nodes Graph Tree. For example, if there is one Serial Node connected to an outside Node, with a alpha connexion (dotted line) but no usual Node connexion (straight line) between the 2 Nodes, Resolve seems to show a logical trouble in the tree with the icons. But I did not yet understand whether this is the only reason why the icons are displayed in the Tree Nodes Graph rather than the usual Preview, or if there is another problem in my logical hierarchy.
I already spent too many hours and the Resolve’s bugs don’t help (specially, the fact the previews don’t always be updated as soon as you modify a parameters…) for a newbie like me, so I will probably follow your advice and restart my tree Nodes from fresh copying the rotoscoping powerGlove data.