This actually just happened to me. I spent over 1.5 million brass and several hours farming brass and upgrading artifacts for one of the missions.
I had to get 6 artifacts to level 12 and equip them to ATK champions.
I accidentally sacrificed one of the champions because he was a rank 1 fodder unit I was using just to complete the mission. Needless to say, I was surprised that none of the artifacts returned to my inventory.
I don't see anywhere IN GAME where it says that equipped artifacts will be deleted if the champion is sacrificed. Being that I did not mean to Sacrifice this champion I wouldn't have tested this on this one, specifically, but now that this happened, it is EXTREMELY frustrating. Adding some code to return equipped artifacts to the inventory is pretty simple to do, and wouldn't take a Developer more than a couple hours to do. And you guys are/have some really good Developers.
This is very frustrating and I feel like I wasted all this time doing something that not even a warning pop up reminds you to un-equip. Literally no warning at all. Frankly, I could understand no warnings if they un-equipped, but they don't. I literally screamed "WHAT?!?!" at my phone when I realized what happened.
Maybe the point was to make the game unforgiving? I mean, I guess. But the game is already a chore to do most things, and that's okay as it's an RPG. This seems more like a punishment and that's not right. I literally feel like I wasted my time now which makes me weary on future play. Sorry, I am just being honest here. I shouldn't feel like this game wasted my time. You even get XP from losing battles. Deleting equipped artifacts when sacrificing a champion seems absurd.
Maybe don't even let sacrificing equipped champions even be an option if artifacts are also "Sacrificed" with nothing to gain from them.
EDIT: Let me add that I wouldn't care so much if the game forced me to either un-equip the artifact(s) or charge me the brass needed to un-equip it when sacrificing. Outright deleting the artifact(s) is the issue here.