I purchased 6 demon wards and six demon slayers from my stronghold they all stacked in my inventory as a stack of 6 in for each different ward. I randomly equip one of the 6 in for the first boss of mDWP. What I noticed was that on after any use in combat the slayer or the ward no longer stack so after using 4 in random order for boss fights they all had different lifetime remaining before they expired and they did not stack any more but the 2 of each that I had not used at all which each had the identical full 2 hours remaining lifetime did stack. (see attached photo)
So my hypothesis is that the bit of code that decides whether wards should stack not only looks at the unique item id/name and the item type (e.g. Bound Character or Bound Account) but is also looking at the time remaining before the item expires and if any of these characteristics are different the items will not stack. I have also found this to be true for other wards like those gained from Hellpit that have a time based duration.
I believe this classification of items as different (and therefore not stackable) if they have differing lifetime remaining to be a bug. The hypothesis should be easy to test and if it holds true home in on the code causing the issue