I don't know why it does it and when it does it but if you download playstation plus stuff and your not a plus member, the game will work for so many days ( to my knowledge its many many days, can't give an exact number) but then you will one day go to load the game up and it will tell you something about game is no longer available and you need to sign into the playstation network , now I don't know if this has to do with time, how many people logged into that account or what my best guess is, Lakkus maybe you didn't renew plus for a month or so? because this happened to me with the plus games you gave to me along time ago but I didn't even say anything to you or ask about it at the time, but when i recently got onto your account I got those games again and they work again, so who knows?
PS+ members get to keep whatever they get for free until they cancel their PS+ membership
I don't know how it works with stuff you buy on the discount though