@Erin Vargas is it possible you have items pinned in multiple workspaces, or pinned objectives? The 15 item limit is the total pinned items for your user across the entire subscription, but when you see the My Pinned Items list it will only show you the items that are relevant to your workspace. Also, Pinned Objectives are shown in a separate section of my work items but they also apply to the 15 item limit.
Regarding the sorting, are you observing that there is a pinned item missing from the list when you sort in descending or ascending and that item exists in the list before it is sorted or is one item always missing from the list?
Kristy