Hello,
I've just checked this myself on official 5.0.d release and SOLR4. I can't reproduce your issue - my custom tag can be used to query documents after SOLR has indexed the document (5-15s after modification). The tag list in document library (I used Repository view) is also updated after adding a new tag.
The fact that taggingService.getTags(store) returns an array of length 0 is an indication that the tag hasn't actually been created. The getTags operation does not rely on SOLR and primarily executes a direct query against the database via the NodeService. The only part of that operation that would use SOLR is the query for the classification root, i.e. the container for all tags. If that already can't be found, you have a major problem with your SOLR setup, because that root is created when Alfresco first starts up. You can check this query yourself using the node browser - simply query for PATH:"/cm:taggable"
Regards
Axel