I can repeat the experience, I had about 7 acousticremains and 2 lighthouse uses left. By usage, I should have lost them in a couple of months, not now. Only realized however, or it happened recently.
Edit: While the lighthouse may have been old enough to vanish, I refreshed the spell with acousticremains within a few months- clearly not exceeding 6. And now that I think of it, I used a lighthouse like 3 months ago.
I assume this happens because it only remembers the first imprint. It might be useful to show when they will vanish, or make them vanish by simply depleting the casts- it is a bit annoying to see it be gone totally, not even remaining in the spellbook.