Unique/Gated Items still Available after Purchase

I've had this problem a couple times, but I'm getting better at checking prior to purchasing.

There are a limited number of things sold by heart vendors, merchants, or random NPCs that are either gated or only need to be purchased once per account.

Why can't these items that are bought once per account (ie permanent home instance nodes, portal scrolls, and collection items) be removed after the sale? Some are removed, but not always.

While buying funerary incense from the Primeval Steward a few days back, I got my regulars and one of the selections went away but the other remained. There was some other game-wide deals going on and I didn't think anything of it when I "purchased" five more. But they did't show in my inventory even though I paid the required price. There was not note that popped up saying I had already made the max purchase for the day.

I've lived with it this long, just wondering who else has had similar thoughts.