I don't recall anyone mentioning this "feature" of the bidding process. While min bidding amount changing and cabin categories being added (after initial invite) have been discussed previously, I noticed that some cabin categories were removed since first being able to bid.
75 days out, got the invite to bid and all categories from just above existing cabin all the way up to Royal Suite was biddable. Sometime in the past two weeks, when clicking on the original invite, all categories above two major ones over existing category have disappeared.
So, with a balcony, the bidding is limited only up to a GS, whereas before bids were available all the way to RS. The existing bids on categories which are no longer shown as biddable (like OS) are still in Pending status, but, clicking on the Modify link, leads to a blank page.