Some of my games disappeared from origin

@crea7uraSimple.
You "bought" the game on a tier seller like G2A.
The original owner of the code, who bought the a physical copy of the game back in the day decided to redeem his code on his Origin account to have it available in his library without having to use the CD. The person saw that his code was used, contacted the support and proved that his code was stolen (because most of the codes are stolen on these websites), and since he his the true buyer, it's a simple order of priority and he just got his game back.
It happens that sometimes the codes you buy on tier-sites are already bought by someone else who has a physical copy of the game and can prove that the code is his. But if the game is not available physically, if the serial is not used, it can't be bought twice and have one of the owners taking it off the other since you both bought it the same way, though it can only be redeemed on one account. The games you buy on the Origin store just generate a new specific serial. I hope that makes sense.
That's why I would recommend buying games from the retailers.