Is it normal
It is not normal for my Rainbow but if the first owner did not change the water,clean the separator or picked up plaster dust or some type of non wettable material yes it would be normal. As the second owner who knows how it was cared for by it's first owner.
I have seen hepa filters that have been in for several years and are perfectly
clean but also seen pictures posted on this forum of machines brought in for service that are caked with filth.
It is a shame cutomers buy expensive Kirby's,Electrolux and Rainbow machines and abuse the machines.