Jessa Wedding
So I don't know if this has been done before, if it has, my sincere apologies.
I think the reason that no one remembers the Jessa wedding is because Ragnor Fell was part of the wedding and if he was, then the warlocks would have to erase Ragnor from the memories of everyone who attended the wedding.
Considering how tedious that process would be to intricately remove one particular person from the memories of all attendees and dangerous because it requires a lot of intricate spellwork and one misstep might drive people insane.
It makes a lot more sense to simply erase the entire event from memory.
But whhhyy was Ragnor in hiding for so long? He went under-the-radar for more than 5 years. Why?
1. He was helping Jessa in the hunt for The Lost Herondale and First Heir of Fae.
2. He was bored with politics and wanted to take a lengthy leave from all Shadowhunter and most Downworlder business.
3. He was working on discovering some sinister thing that we haven't encountered yet. Maybe something related to Magnus' dreams of the world being drenched in blood (don't remember the exact wording).
4. Maybe it was a warlock ploy to get the Shadowhunters to listen to Downworlders more what with 'look a warlock-employ of the Clave was killed by one of our own maybe we should go easy with our racism lest we have a full-blown revolt against our corrupt practices'. But that doesn't seem grand enough a reason to just dupe the world for 5ish years.
5. Haha let me dupe the entire Shadow World this prank is lit.