Very annoying bug. If I used the last siege for that quick slot, and stow it back, the siege in that quick slot disappears and i have to re slot it. To make matters worse, I can't reslot it back during combat
I tried to reproduce it but it seems to be a random bug. Could be an add-on as well:
How it happens:
1) Slot one siege in quick slot (make sure you only have 1 of it)
2) Go cyrodil
3) use the siege
4) WALK away from the siege
5) use it, and then stow it back
6) if the bug triggers, siege disappears from quick slot