Unknown tag in finder
If the files you have issues with are self-created simple files like the text file you depicted, you can run a Terminal command on those files to clear out specific or all extended attributes.  This will effectively remove ANY tag you attached previously.  Once done, you can re-attach tags in the fashion you already know.

Keep in mind that some applications may use extended attributes to complement the data inside a file.  So, deleting all attributes is not always the best idea.  For simple (text/image) files, it should be ok however.

Here are the commands to clear the responsible extended attributes:

xattr -d com.apple.FinderInfo /drag/your/file/from/Finder/here
xattr -d com.apple.metadata:_kMDItemUserTags /drag/your/file/from/Finder/here


Here is the command for clearing ALL extended attributes on a given file:

xattr -c /drag/your/file/from/Finder/here


To know what extended attributes are attached to a file, you can run the following command in Terminal to list them:

ls -ld@ /drag/your/file/from/Finder/here


Output from that last command will look something like this:

drwxr-xr-x@ 2 youruser  staff  68 Apr 19 14:28 /your/file/from/Finder/here
    com.apple.FinderInfo    32 
    com.apple.metadata:_kMDItemUserTags 52