ShellTile.ActiveTiles sometimes forgets about one of the tiles

A couple of times, when working with an app with lots of tiles pinned, I've seen `ShellTile.ActiveTiles` "forget" about one of the tiles.  By that I mean that when updating all the tiles from a background agent one of them isn't updated.

This is with code like:

    foreach (var tile in ShellTile.ActiveTiles)
    {
        // update the tile.
    }


At the moment I only have anecdotal evidence of this behaviour but wondered if anyone else could shed any knowledge or experience of this while I continue to investigate.

Deleting and repinning the tile in question seems to make the problem go away but this isn't a real solution.
Additionally it doesn't seem like it's the first or last created pin but I can't be sure of this.

Has anyone else seen this?