I heavily depend on tags. Currently you can only tag files that are already indexed. This is a problem when working with folders that are not indexed. You can tag any file that is present in an unindexed folder, but that file will not show up in your tags unless you index it first. Most often, for a project, I need 1 or 2 files from my archive folder which is not indexed. If I need to work with tags, now I need to index the whole archive folder. As the folder doesn’t contain files that I always need it will just clutter up the file search in menu and make it harder for a user.
Tags are generally just extended attributes set on the file (Try running this command in a terminal: getfattr -n "user.xdg.tags" FILENAME), and as you’ve noted - you can definitely set tags on unindexed files, and if you browse to that file (using Dolphin or any other viewer that supports reading the user.xdg.tags extended attribute) - they will show up.
The issue is what you mean by “show up in your tags”? I’m guessing you mean that you can open Dolphin and navigate to the tags:// view and see the tags that you’ve used, and when opening them - you’d see the files you’ve tagged.
Well, the Dolphin “tags” view is a Baloo database search - the system KDE uses for indexing. There’s basically no other way to do it - except to run a full recursive directory scan of your disk every time you open the tags view - which you would not like at all - so it is what it is: the “tags” view only shows files that were indexed into the database because that is what it shows.
If you want to find unindexed files tagged with specific attributes, on a one off basis from time to time, you can maybe use a dedicated tool for that - unfortunately I don’t know any graphical tool that does this, but maybe you will find the following command useful:
getfattr -R -n user.xdg.tags path/to/folder | grep TAGNAME -B1