While some of it seems indeed like its bugged, at least for MOM the explanation is, that not all the cards show up in packs. There are 5 commons, 5 uncommons, and I think 5 rares, that are not in packs. You want those, you have to craft them. Those cards are from some other MTG products that technically belong to the MOM set, but are prints for those products only.

Using MTGa on Windows (10) and Android (10,11,12)

MOM/MUL/MAT packs have been giving me gem rewards for a while now even though I have not completed any of those three sets. I must have opened well over 300 packs and played between 10-20 drafts which is usually more then enough to collect at least all common and uncommmon cards and I even bought over 30 packs to complete those sets and received only 5 rares/mythics and what's worse: I still have not completed the common and uncommon cards!?! The following cards I only have 1 of in my collection but I can not seem to add more through opening packs. I do not want to waste wildcards to complete recent sets.

Zephyr Winder
Phyrexian Pegasus
Fairgrounds Trumpeter
Injector Crocodile
Cragsmasher Yeti
Seedpod Caretaker
Referee Squad
Axgard Artisan
Essence of Orthodoxy
Interdisciplinary Mascotte
Terror of Towashi
Orthion, Hero of Lavabrink
Surrak and Goreclaw

Not sure, but I think they are all cards from mastery-rewards of the MOM set

TLR gives a similar issue: also gives gem rewards when opening packs instead of missing rares/mythic cards. Have opened at least 30 packs in the last couple of weeks and received no rares at all.
TLR does differ slightly though: when I look at my collection in profile for instance, it says there are 4 common cards I have not yet collected.
But when I go to my collection and filter out collected cards, only the nazgul card show up, which I am 100% sure I have collected way more then 4 times allready. I even tried to use wildcards for it, (several times, wasted 7 UNC wildcards on it) but everytime I craft and leav collection the number of nazgul cards drops down to 1. Where are the other 3 uncollected cards? And how to complete the set? Again, I do not want to waste wildcards on it and have been grinding events to no avail.

Update1: Tried opening a goldpack and it gave me 5 gem rewards and only 1 Mythic card.
Update2: MOM uncommon Tried crafting axgard artisan from 1/4 to 2/4, seemed to work, but did cost another uncommon wildcard.
Update3: MOM common Tried crafting Fairgrounds trumpeter from 3/4 to 4/4, seemed to work, but did cost another common wildcard.
Update4:
Update5: TLR common: Tried crafting Knight of the Keep
Update6: TLR uncommon: Nazgul shows as 2/4 collected but in craft window shows only 1/4 collected, have previously allready tried craft to 4 on 2 seperate occasions which seemed to work but after re-entering collection the crafted ones dissapeared leaving me 6 cuncommon wildcards lighter, please reimbourse those asap.

This has happened to me twice today.

Bought 10 packs of All will be one. Received gems for 7 of the 10 packs. Have not completed all rares or mythic rares. This has happened multiple times with this being most recent.

It is likely a copy of Tibalt's Trickery