Reading more posts by Peter I now believe that FD can really be lowered on a router being in a Passive mode:
"Feasible Distance, or FD, as we have discussed already, is a record of the smallest CD for a particular destination ever encountered since the last time this destination went from Active to Passive state. As opposed to RD and CD that are kept for each neighbor to a destination, FD is kept only for the destination - so regardless how many neighbors you have, a router maintains only a single FD value for a particular destination".
"Feasible Distance (FD), holding the lowest known value of the CD toward a particular destination since the last time the destination went from Active to Passive state".
You see that? It's not about computing an FD when being in an Active mode, it's about keeping track of the lowest encountered CD when being in a Passive mode and calling it an FD.
So, when router is in a Passive mode (not sending Query packets) it still can compute distances towards destinations and once the lowest CD appears (as compared to the current FD) it becomes a new FD.
I also recall that Brian explained that FD can be lowered, but not increased. Being in the Active mode a new FD can be higher. But being in a Passive mode (Update packest being sent) FD can only be lowered.