This article claims the following:
The second limitation of potassium sorbate is the length of time it is effective. Once added to wine it stays in the desireable form of sorbic acid only for a short time. Over time it breaks down into ethyl sorbate which can add notes of pineapple or celery to your wine. The change into ethyl sorbate is not preventable. By using potassium sorbate winemakers are putting a definite shelf life on their wines before they pick up these off flavors.
I wasn't able to find any more articles that verified that potassium sorbate causes off flavors over time, and, more importantly, how long it takes for potassium sorbate to degrade into ethyl sorbate.