On the 27-inch iMac, available storage upgrades have varied depending on which stock configuration is used as the starting point, with the low-end and middle-tier models allowing customers to select from 256 GB, 512 GB, or 1 TB of SSD storage as upgrades from the standard 1 TB Fusion Drives, while the high-end model allowed for upgrading to 512 GB, 1 TB, or 2 TB of SSD storage from the standard 2 TB Fusion Drive.
As of today, however, the available SSD options on the 27-inch iMac lineup in Apple's online store are much more limited and somewhat conflicting depending on the starting configuration. On the low-end model, 256 GB is now the only SSD upgrade option available, while on the middle-tier model Apple is showing no SSD options at all. For the high-end configuration, only a 512 GB SSD option is available.
While the change is consistent across Apple's online stores in multiple countries, the Apple Store iOS app continues to show all of the usual SSD options.
The 21-inch iMac is also unaffected in either the online store or the app, with Apple offering up to 512 GB or 1 TB of SSD storage depending on starting configuration, and it makes little sense for the 21-inch model to offer better SSD options than its larger sibling.
As a result, this is very likely a bug with Apple's online store that should be fixed relatively shortly.
Update 4:15 PM: Apple says this is indeed an error and is fixing the issue.