It's something I have noticed in all the groups that not only have I run but also Deviantart in General.
It's a glitch that has never been fixed that automatically removes people's artwork from the group after a while. The mods in their "eternal wisdom" would never heed my complaints about it.
Keep in mind, I hardly even come to DA these days and it automatically seems to have removed it. Call it Bullshit, whatever. but something's up with the Group coding.