Not sure where this needs to be fixed, but you cannot get imprinted crystals for mobs that have variants if SpecialMobs is installed. I don't expect the variants to work, but the vanilla "variants" should.

Affects as far as I have tested ender men, skeletons, ghasts and zombies, probably more.

SpecialMobs support has been added.

The "vanilla" variants are modded mobs replacing the normal ones, which is why they don't work by default. I can look into adding support for them in the future, but for now, you can enable the "Wrath Cage Cries Havoc" config and they should be able to imprint.

Is this the same reason that the mobs don't appear to drop essentia to the cursed blade? Will it also fix that problem?

Possibly? I don't know what the cursed blade is, but Special Mobs mobs won't have any aspects on them unless the mod author specifically adds that functionality on his end.