I've had this issue for a while but I wasn't entirely sure but I've just confirmed it now. I collect cards from boosters that dont actually end up in my inventory. I confirm this when I collect a SECOND copy of said card in a later booster. E.G. Zombify from the new Strixhaven. I opened a copy on Arena from a reward booster last week with the new mystical archive treatment but my inventory didnt update. I then aquired a 2nd copy just now from a booster on arena, this second one also noted it was a 'First Time!" pull and it Finally updated my inventory. This has been going on a LONG time but it was hard to pinpoint an exact exmaple because I didn't believe it was happening for a while and when I did start to notice I played infrequently.