Humble Bundle Subscription Question

The state has changed. The expiration date for a slice of toast may be different than a slice of untoasted bread, for example.
Within digital interfaces it's actually more common for [different state] elements to have changed parameters than not, though in many cases the state doesn't actually change until the process is completed, meaning the presented state change is only a facade.

Put more clearly, it depends on whether Humble is still viewing it as a coupon or as [redeemed]. In the latter (and more likely) case, the intuitive interpretation would be that it has already associated with the active month's bundle content (and may even expire relative to that bundle, though as it's a standard purchase format for the site it's not likely to do so).

Less likely possibilities include "a different expiration date" or "links to whatever bundle is active when you finish processing the link". Of course, it's still possible- as you suggested- that the state hasn't changed, and that it expires along with the original coupon's promotion period. The OP's enquiry on the matter make sense, however, as that's not the only possibility.

(Note that it's not a gift link for the coupon, the coupon was already redeemed for a gift bundle.)

I tried the link and it says that I can redeem it for this month's bundle if I claim it, which is weird.