Brainwashing only works on a broken mind.
A general said "we break them to build something better".
Thats why the higher tier soldiers are more deep in their ideological brainwashing as well as having been subjected to more difficult training than the basic tier.
A mind breaks when the things they placed their faith in, falls apart. Family, community, religion, political persuasion, etc.
And in that broken state, something new can come in. Every testimony of FORMER Atheist, Christian, Muslim, Democrat, Republican, etc. has that common denominator.
Meanwhile, those in the mentioned groups that have a solid foundation in their family, community, etc... dont become a "former" no matter how convincing the foreign argument sounds.