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Hey, I am sorry for late reply. Did this fix itself? It is possible that if you do a huge sync from nothing, you might not see tags until the next client reboot. You can check this by opening a new search page, changing the tag domain from 'all known tags' to 'public tag repo', and then doing a search for system:number of tags>0. Does anything show up at all?
The PTR in hydrus should just make tags appear, you don't have to do anything but wait for some syncing. If you check the PTR's page under services->review services->remote->tags->ptr, how far is the 'processing' stage done? Is it mostly finished? It should have a mappings count, and as soon as you get, say, 50 million mappings, you should see a bunch appearing on older popular files.
For some situations, tags will not line up with a file. A couple of hydrus users had run their files through image resizers/optimisers years ago, and since the file content here has changed uniquely, those files will not line up with the (hash, tag) pairs the PTR offers. But if you have any decent sample of original files from the big boorus, you should see some tags from the PTR.
Here's a picture of Samus:
https://safebooru.org/index.php?id=1919822&page=post&s=view
If you select and drag and drop that URL onto hydrus, it should import into a new page. The file has tags on the PTR--do you see them?