Steam Group don't show in SG

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I'm actually no longer finding several of my steam groups on the groups page [though they were there in the past, and are still active on Steam], but they're still showing up for me in the giveaway creation group selection. Not sure if that's related to your issue or not, but maybe it'll help narrow down the issue.

Seems like no one reading this has an answer so far. I don't have an answer for your problem, but maybe I can assist a little.
The groups you are able to create giveaways for should AFAIK be listed in https://www.steamgifts.com/account/steam/groups, but as you say, Adult Only Games doesn't show up there for some reason. I tried joining that group and syncing SG manually to see if I had the same problem and indeed I do. Steam lists all of my groups both on my steam profile and at https://steamcommunity.com/my/groups/. SG lists all of my groups except Adult Only Games. I even joined a group with the term "adult" in it and created this new test group:
Group Name: Adult Only Test Abbreviation: Adult Only 2 Group Link: [...]/adult_only_2
Both groups are detected by SG & I can create GAs for them, which leads me to believe there's either a glitch in SG's system of detecting groups, or your group has been added to some filter list. I don't know how it all works, that's just my personal reasoning.
This might be irrelevant, but I noticed you're the group owner and that your Steam game details are not public and that the other admin (and officer) of the group has a private profile. Maybe try changing to fully public, syncing your SG profile manually (you might have to wait a while and then update again - some times SG needs some time to sync with Steam). Just a suggestion.

An idea. Try to create a giveaway for the group, even if it doesn't appear in your group list, to see if at least it works.

Just out of curiosity, did you find out the reason for this occurrence?

Hey I have the same problem with a Steam group I just created one hour ago, and a few successful syncs since then has not resulted in my new group appearing in my list on SG or when creating a new GA.
My group is listed on my profile as the "Primary Group" - I wonder if that makes a difference?