Same exact problem.
The guild bank knows the items should be there. For example, bronze-quality Saxifrage. I have 132 in a slot. I have 1 in my inventory. I right-click the 1, but nothing happens. The guild bank knows there is a slot, so it won’t automatically create a new slot. I can manually drag-and-drop to a new slot. I cannot drag-and-drop or right-click into the pre-existing stack. I cannot remove or move the pre-existing stack. I cannot split the pre-existing stack into separate stacks (though it knows I should be able to, as it opens the stack split window and allows me to try, it’s just it doesn’t actually DO it).
I don’t know if this is a guild bank issue or an item issue. It is occurring for a variety of items in multiple tabs, and a variety of slots within those tabs (not the same slots in the grid either).
My husband and I both tried from our separate battlenet accounts. I tried from my main (the guild leader) and his character is a guild officer. We do not have any addons that impact inventory. We have not tried removing addons or reinstalling, but it looks like that doesn’t help anyway so not going to at this point.