As far as I can work out, I may be wrong (as is usual haha) but I believe that more senior members of certain legions are allowed to keep their knowledge of the Grey Knights.
Logan Grimnar (of the Space Wolves) is very much aware of the Grey Knights and has been allowed to keep hisbmemory of them.
I also think I remember reading that the reason that people have their minds wiped after encountering the Grey Knights is not so much to keep their existence a secret but to erase the trace of any demonic rituals encountered from the minds of the corruptable. I also think it may have something to with the tech and rituals that the Grey Knights use, they are borderline heretical in nature so they may mind wipe to erase knowledge of heretical practices? Maybe if certain beings don't see these practices they don't neccesitate a mind wipe?
It's also worth remembering that the sisters of silence are amongst the Emperors most trusted servants, they, along with the Custodes, are the only people given the freedom to walk the inner halls of the Imperial palace.
They also are sworn to silence, who they gonna call? data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
It probably has more to do with the inconsistencies of the older and newer lore/fluff. Different authors as well as different codex writers have different standards to how closely they follow lore, one in particular is maligned by the table top community: Matthew Ward. Ward tends to take a big dump on established lore when he wants to make someone/thing sound cooler, an example would be something I read years ago on Boltersandchainsword.com
A user called Ragnar_Blackmane had this to say about Matt Wards massive exagerations: "the fact that only one of 1 million neophytes shipped to Titan becomes a Grey Knight (if that was the case they wouldn't use whole delegations to hand pick recruits and would have to take every single psyker the black ships find no matter their sanity/power level to get such an insane amount of psykers together"
I believe he makes a good point there.
I find Luetin09 on YouTube is very helpful with lore/fluff inconsistencies, I may well pose your question to him (unless you want to?) As he will provide a far more detailed answer than I can as he seems to have the ability to cram all the lore into his head, I struggle to remember my own name data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
Cool. You can go ahead and Luetin09. Heck, let's also ask Remleiz from 40K Theories on Youtube. As for Matt Ward, maybe he's being paid by the Inquisition to make those inconsistencies lol.
I love reading books, but time to time I back to those I have read previously. If you like the Harry Potter series like me you will love the Hunger Games series. It is a trilogy and contain: Hunger Games, Catching Fire and Mockingjay. Also I really like Divergent (it is also a trilogy containing Divergent, Insurgent and Allegiant). It is a science fiction book so I hope you will like it! If you are into horror books I recommend Full Dark No Stars written by Stephen King. It was really mysterious and sometimes brutal. Let me know what do you think!
Since my late mother read it to me when i was young i really got into love story of Winter for Two by Nicholas Sparks.
Shantaram, The Shawshank Redemption, The Kite Runner, 1984, Martin Eden
3 books have made me cry
The Stand Stephen King (you all know this one)
Anniversary Man R J Ellory (crime fiction - brilliant writer)
The City Of Mirrors Justin Cronin (3rd book of a trilogy - first is called The Passage - post apocalyptic)
I see this has just started up as a Fox TV series.
Yeah and it’s pretty good but too slow. There are 3 books that get progressively bigger and the story is epic. However I’ve read that they’re taking the whole first season to do 25% of the first book - which is waaay too slow do they think they’ll get 10seasons? I enjoy it but I’m patient because I know how amazing it could get. Not sure a non reader would feel the same.
Not sure about the best but Things Fall Apart by Chinua Achebe stayed with me.
The Unbroken Chain by Guenter Wendt and Russell Still.
Guenter Wendt was the launch pad manager at Cape Canaveral from Mercury through to Apollo.
These kind of books are dry at first then unfold and you begin to get an idea of what when on away from the cameras, bit of a humbling experience but that is when you get an idea of what people can achieve.