I think I may know how this happened.
See, it appears that Brainstorm wasn't always intended as a Walgreens exclusive.
Check the numbers above his UPC barcode. That's his own unique product number, followed by the assortment number for the general retail Titans Return Deluxes (B7762). How do I know that? The same number also appears on the packaging for all the other general retail Deluxes, such as Hardhead (wave 1), Chromedome and Highbrow (both wave 2). See the following comparison:
Now some of you may ask, what about the Combiner Wars "May Mayhem" exclusives? How did they handle it?
Here's last year's Quickslinger and Brake-Neck, both of which only had a product code number but no assortment number. The general retail Deluxes all had the same assortment number (B0974), such as Rook (wave 3), Sunstreaker (wave 4) and Trailbreaker (wave 6).
And because those all came in English-only packaging with comic books, here's a comparison for the multilingual packaging format as well, featuring this year's "May Mayhem" exclusive Groove (no assortment number) compared to general retail wave 1's Alpha Bravo and Dragstrip (both also sporting the assortment number B0974):
So yes, it
is unusual that an exclusive sports a general retail assortment number, which reeks of a change in plans very late in the process.
The last time this happened was Age of Extinction One-Step Changer Stinger, sporting the general retail One-Step Changer assortment number (A6151), as did wave 1's Optimus Prime, wave 2's Crosshairs and the final wave's Prowl.
Stinger was officially supposed to be a Walmart exclusive, like all other AOE Stinger figures, but he was actually found at multiple Target stores, complete with the "Only at Walmart" sticker.
So long story short: Making an intended general retail release an exclusive at the last moment is very likely to screw up logistics.