GitHub didn't pick up tag v2.21_itempack as the highest semantically. While you've marked the release as the latest, it's been automatically placed below the release tagged v2.2_itempack due to some quirks in the way GitHub sorts releases. More info here. As a result, modhub is incorrectly detecting v2.2 as the latest (i.e. the top one) and this is the version the download page on the forums is still pointing to.
Could you try refreshing releases v2.2 and v2.21 by clicking Update release without making any actual changes? If this doesn't help, try re-releasing with a tag named "v2.2.1" to see if this is a problem with tag version semantics.
Nothing really worked until I set the previous version to "prerelease", that seems to have done the trick. That will teach me not to get cute with version numbering.
Many thanks for the heads up.