UPDATE - As of Patch 6, the Misty Step trick no longer seems to work. And there are more issues with the gems disappearing when inserted into the altar. This may only be an issue for PC players as I did not experience that on console when I did this recently. Also TYSM to everyone for all the feedback. ♡♡♡
•••••••••••
HI BG3 players!! SO. This is a known bug that I've seen multiple posts about on Reddit and other corners of the internet. MY particular issue was slightly different. I was able to get enough gems to unlock this door normally...BUT my library area was bugged which prevented me from obtaining Shadowheart's spear. This is essential to continuing her story. If you try to get through the Gauntlet without it, she will end up in combat with you.
So I reloaded to a save where my Library area wasn't bugged and I could obtain her spear but not have enough umbral gems to open this door. This is precisely the glitch that some people are having. Thanks to a Reddit user, I was able to utilize Misty Step to get Astarion behind the arcane locked door and pull the lever to let everyone in!!!!
TLDR; you don't need all the umbral gems to get in here. Just use Misty Step!!! Hope this helps.