Menu block selectively disappearing for a single Views argument

See: http://www.growingupguidepup.com/gugp

A view exists, with a path of gugp/%, "%" being a numeric argument specifying a season number. Menu items have been manually created for season 1, season 2, and 'extras' (season 0).

On "season 1" (gugp/1) and "extras" (gugp/0), the menu block appears.
On "season 2", it does not.

The menu block is set to be visible on all pages (and is used without any issue under "The Puppies" as well). Season 2 was recently added and for some reason that I can't quite fathom, this menu simply will not appear.

As far as I am aware, nothing particularly special was needed for season 1 & extras.

Note that the menu is active and working on episodes within a season, suggesting the problem is somehow related to the "view": http://www.growingupguidepup.com/gugp/2/201

Where else might I check to determine why this is not being shown on the "season indexes"?