What if Trick erased her memories for whatever reason? Whether to protect her, by her request, or for some more malicious reason. Perhaps it did have something to do with the wanderer, considering we're meant to get a look at him this episode, and that very well could mean in a flashback. "Why do you care so much!?"
Hmm... Trick could have seen the future or something, found Tamsin, tortured her, and killed her, in hopes that would erase her memories of her mission, and thus him hopefully "saving" his future granddaughter? That feels like something right up Tricks ally. Walking the line between good and bad, excusing his evil actions with what "good" he hopes to achieve.
So have I. And that he is the Wanderer too, because of the caption, and the fact that you can't see his face.
Because he's there for a multi-episode arc starting with episode 9, don't know if he's the Wanderer but the timing plus secrecy seems to point to it being him. And I'm pretty sure that's him in that picture Rachel posted.
On the subject of why Tamsin doesn't seem to remember Trick, I was thinking maybe in the past she met the blood king but never made the connection with him. I mean, since the beginning of the series no one gets that Trick is in fact the blood king even though he must have been the most famous fae at one time. He had to show Dyson his arms for him to know who he was, the former Ash had no idea either etc etc. That doesn't explain why he never acknowledged knowing Tamsin though.
I'll have to rewatch season 3 since I can't remember exactly, but to me it seems like Trick was the only character who never expressed negative feelings upon Tamsin's arrival. I mean she wanted to put Bo behind bars and throw her to the Dark ! Dyson growled a lot, Kenz was not feeling the Tamsin Love, Bo was wary and even said they kinda hated each other. I don't know, but one would expect a little (lot) more animosity towards someone who wanted his granddaughter's head... Unless... Unless he knew something the others didn't.
I don't know, maybe my brain went kablooey after the last episode.