Cannot get this working in Typescript (using neovim)

[ukch]

I would like to get syntax highlighting for an xml syntax. My file:

# custom.xml.ts
import { xml } from "lit-xml";

export default xml`
<some>
    <xml>
        <content>
           ...
        </content>
    </xml>
</some>
`;

In my ~/.config/nvim/init.vim I have the following block:

...
Plug 'cdata/vim-tagged-template'
let g:taggedtemplate#tagSyntaxMap = {
  \ "html": "html",
  \ "xml": "xml",
  \ "md":   "markdown",
  \ "css":  "css" }
autocmd FileType javascript,typescript : call taggedtemplate#applySyntaxMap()
...

When I first open this file in vim, the xml portion has no syntax highlighting. Manually running the command call taggedtemplate#applySyntaxMap() has no effect.

However, if I do set ft=javascript, then run the applySyntaxMap command, the highlighting appears correctly (NB: the autocmd doesn't seem to work - I still need to run the command manually)

[wgxh]

What?Why i can?