Siege in the Guild Bank
That is the same for everything that stacks, not just sieges.
Why should the guild bank work differently than the personal bank? Was this intended for some reason? Maybe because there will only be one person at a time accessing the personal bank, whereas two or more people could be trying to add to a stack of items(at once) in the guild bank? I’m thinking this might be a deliberate “fail safe”, rather than a bug? Hopefully someone from Anet will clear this up.