Can also confirm, this has been the case for a few days now. Can only use “Stack All” inside my item hangar and in ship’s cargo holds, but no longer in corporation hangars or any containers. Makes sorting items a real pain. You can still manually stack items inside containers or corp hangars by dragging one stack of items on top of the other one, but if you have hundreds of different items in a container, that is really cumbersome.
So either this is a bug and CCP didn’t fix it yet, or they disabled the feature for some reason (e.g. performance) but didn’t tell us. If it really is a performance fix, I just don’t get it if it is less overhead for the database to just perform the stacking operation than moving a hole bunch of items into a different container/hangar, then perform the stack, and have the database change all the items back into their original container. That simply doesn’t make any sense, so more likely it seems to be a bug.