Thanks for the report, StarManta!
What you describe is actually the correct behaviour. That is one of the consequences of the September 2019 errata on "Shapeshifters" and works the same for other cards that can be trashed for some benefit (using clauses like "If you do/did" or "trash ... for ...").
You can find the full text on forum.dominionstrategy
, but I'll quote the relevant "stop moving" rule.
The stop-moving rule: An effect can move a card if it specifies where the card is coming from, or if the effect put the card where it is now. If a card isn't where the effect would expect it to be, or has moved away from there and then back, it can't move the card. Played cards expect to be in play; they can't move themselves if they aren't. Gained cards are expected to be where they were gained to, even if this isn't the discard pile. Cards in discard piles can be moved even if covered up by other cards; cards on top of a deck can't be moved once covered up.
So the Pixie expects itself to be in play and can't trash itself for the double Boon as in your case it isn't actually in play (it was set aside when you bought Inheritance, and the new text of Estates makes sure that each time it is played it stays there).