There is none
There is only one other mention detailing the Desert's Night in all of Planescape, or D&D 2e's book, in Planes of Law: Baator, page 21:
Ankhwugaht's fields produce the intoxicating blossom known as desert's night, a sedative said to set one's mind wandering. For an exceedingly high price (usually about 1,000 gp a blossom), a traveler can purchase the deadly bloom. It's used by sniffing the flower, which requires a save versus poison. A failed save sends the user into raging paranoia, and if he's not cured within 3 days, he goes berserk and attacks everyone in sight until he's cut down. A sucessful save, however, means he spends an enjoyable and profound day exploring his own mind, recovering memories sand healing psychic damages. This is the only known remedy for restoring memories lost to the Styx (save versus spell a t-6; success indicates a restored memory).
Which doesn't say anything about the god using the flower.
The flower is also mentioned as a pit fiend's treasure found in the Temple of Neheod in Hell dungeon from the adventure A Paladin in Hell. It is also mentioned briefly here and there on several other books as "the flower that can restore memories" everytime that Ankhwugaht is also mentioned, but not much is developed on that.
I found no (Egyptian) mythology legends about Set losing (or restoring) his memory either (there is no such thing similar to the Styx in Egyptian myth). That tale makes little sense and must have been an adventure hook for GM's to work with when necessary. So, I believe the author is trying to say that Set must have some reason to be growing the only thing on the metaplanes able to cure the mind wiping effect caused by the River Styx, and he might have needed that flower before, so he is keeping it growing for when he needs it again. But the exact reason is never given to us.