Why did my backpack capacity drop?

When you purchase the Gold Pass, you are granted the benefit to have a larger energy reserve and larger backpack space for the duration of the Trader's Adventure/Gold Pass event.

When the event ends, your energy reserve and backpack space return back to how it normally was.

Of course, you retain any upgrades done while the Gold Pass is active but the benefits are only active when the Gold Pass is active. We can assure you that this is not a bug.