Warning: This question spoils the Horus Heresy, book 29, Vengeful Spirit, as well as information about the Emperor.
During the Horus Heresy, the Sons of Horus and Mortarion's legion invaded the planet named Molech. The goal of this invasion was to obtain something (not spoiling this) linked to the Empyrean.
During his search, Horus tells the Mournival that other primarchs were present here, as well as the Emperor. The Emperor gathered the (discovered at this period) primarchs here in order to make them "forget" (it is not actually true, but it simplifies the context) about how he became powerful.
Just after that, Horus kills the angel that "makes the primarchs forget" about Molech, and Horus remembers.
However, it is not said (at least in this book) if the other primarchs also remembered what they were supposed to forget when the angel was killed, in particular Jaghatai Khan, who was in the cave with Horus and (if I remember correctly) Sanguinius.
Did the Khan and Sanguinius remembered what the Emperor made them "forget" on Molech? If yes, did it have an impact on their loyalty to the Emperor?