Using tags on snapshot to gather cost information via Cost Explorer not working as expected

It may take another 24 hours after tag activation for the tag to show up in cost explorer. As soon as the item is tagged, it will start showing up with that cost allocation tag in cost explorer (i.e. you can do it when you launch your snapshot, or add it later)!

When you add tags to EBS volume snapshots, there might be a delay before they appear in the Billing Console. However, even after you've activated these tags as Cost Allocation tags, they might not immediately appear as options in the Cost Explorer. This delay can be attributed to the periodic updates of the AWS Cost Explorer, and it might require a bit more time than just the standard propagation delay for billing. Regarding your second query, when you add a tag to an existing snapshot, it will not retroactively adjust past costs in the Cost Explorer. The tag will only start tracking costs from the point it was added onward. So, if you wish to have a full historical view of costs for a particular snapshot, it's best to add the tag when the snapshot is created. Otherwise, you'll only see costs associated with that tag from the time it was added to the snapshot onwards.