When will "Uncollected Garbage" be deleted?

This is only mean that the Garbage Collector has moved them to the trash already. This may happen, if the client deleted the TTL data before it expired, and there is an easy check:

this is a bug in the current version. You may restart the node to allow the used-space-filewalker update the trash usage.

You need to use du -s --si instead, because the dashboard showing numbers in SI (base 10).

It’s happening already. Search for retain and Prep|Move.