Ok bear with me,

certain routed have seats blocked off for Flight Crew rest for a number of reasons, some flight Crew prefer to take rest in the cabin instead of bunks if they are not too tired at the times of their break or want to watch a movie then sleep.
This forms part of their agreement, however those seats can be released by Flight Crew in mutual agreement for staff travel passengers in order for them to get on.

seats which are allocated are always the same on The same aircraft type hence they are blocked and no one at the airport can touch them.

they also remain empty for the flight if they are not required for rest or staff travel passengers and even if this is the case, Cabin Crew generally can’t release them for use of other Customers unless all flight Crew agree to this.

on terms of preferential treatment, yes this should have been done more discreetly but maybe they were given F amenity kits and PJs because they were on a FIRST priority but did not get their entitled cabin due to FIRST being full?

I hope this explains some of your points how this has happened but no there is no way around it on certain routes, seats are Crew rest and remain as such whether they will be used.

Same goes for some 777 aircraft which don’t have rest compartments and some the Cabin Crew need seats as there is not enough space by the doors for everyone to rest.

I can assure that Flight Crew would have taken their rest on such a long sector and would have done so in the Flight Crew rest compartment (bunks)