Sorry to necro your thread, but I hope it is okey since I found suggestions of fixes, should it happen again.
I did try to recreate the problem when you posted, with the Singh family in Starlight Shores; but were unsuccessful, and the box could not be moved into Matias's inventory by me, so I did not post anything.
But in another save this happened:
with a family, where the toddler had gotten a IF in the mail that I immidiately placed in the grandmother's personal inventory (I am not fond of the dolls); I moved to another world the EA phone-way, and upon arrival the doll was placed in the toddler's personal inventory (and also named by the game to a word I have used only here on this forum in PMs, so the little IT-clown just got creepier).
I tested if a regular reset would do the same: placed the IT/IF in grandmama's inventory, went to Edit Town, split her from the household and moved her in directly without exiting ET - and when back in game the clown was back in the toddler's inventory. Hurr. Wishes are lost when evicting.
Anyway, I guess the official ownership is the ground for the mechanism and that it perhaps would work the same for the box, it belonging to the career and Priscilla, even if it is stuck in Matias's inventory.
When I tried to recreate the problem and after Priscilla's first show, that I made Matias accompany her to hoping that the box would switch owner's; the box instead got stuck in her inventory. I used the reset above to loosen it.
When trying to solve another problem with the box, I found that placing it on the lot, saving, exiting the game, don’t clear cashes, start the save again - made the game place a duplicate box in the personal inventory of the magician. It does not happen every time. I am not sure it is a recommendable way of getting an extra box, since doing the same thing when travelling (WA), like said above, starts a chain of events you cannot stop - unless there are two boxes placed on the home lot already (and presumably none in inventory of Faustus aka the traveling magician :smiley: )
I found that Priscilla's performance on stage, even with the box, was very bad, and I levelled her up to level 6; so I think that her traits, as well as lack of good ones for the career, like Natural Born Performer, and the LTR Engaging, contributes to that. Carl's has a good guide on the career:
https://www.carls-sims-3-guide.com/careers/showtime/magician.php?cmtx_sort=1
I have played the career once before, and the sim was doing very well, she had traits Ambitious, Charismatic, Natural Born Performer, the LTR Engaging, as well as others, and I followed the tips in the guide.
I am looking forward to your, eventual, update on the box, it is a very interesting magical object :smile: