This was suggested to me through the org-mode mailing list: replace ‘org-tag-persistent-alist’ instead of org-tag-alist in ~/.emacs. I tried it: autocomplete works for both the local and global tag lists.
KO, I think that C-c a m works only in Agenda files and the one I searched is not a dedicated agenda file. Anyway (org-match-sparse-tree) which should do the trick delivers more than I want i.e. every Headline no matter which tag it has. Puzzled.
You should use the new advice system `nadvice` instead of the old one.
xref-find-definitions prompts the user already in its `interactive-form`. So you need to run `open-tags-file` in the interactive-form of your advice for the case that xref-find-definitions is called interactively.
I use the interactive specification of xref-find-definitions of emacs 25.3....
You have a spelling mistake:
The word is spelled function
I'm not using these features so I don't know for sure, but I suspect that the problem will be solved if you fix that name.