Aha, you have stumbled on an odd undocumented limitation of DA, giphy and tumblr downloads–they don't remember tags of queued items through a reboot.
I had forgotten this was true. It was based on a compromise when I rewrote my downloader engine to persist through reboots, in that these sites produce tags at an inconvenient time in the search-download-import cycle, when I can't easily store them for later.
Unfortunately, you won't get tags for any of those queued items. Once your subscription is done syncing, you can semi-quickly recover by running a manual download that mirrors your subscription search with 'get tags even if file is already in db' checked. The client will queue up those files like normal, recognise that it previously downloaded each file in turn, and progressively add the tags to each 'already in db' file without having to make any more requests to DA.
Next time I extend the downloader engine, I will be able to fix this.