Highlighting on handlebars doesn't appear to work

[nionios]

Below is the code for the detection of handlebars files in init.vim (I am using nvim)
On HTML files the highlighting seems to work, and set tf? in nvim returns hmtl.handlebars

let g:mta_filetypes = {
\ 'html' : 1,
\ 'xhtml' : 1,
\ 'xml' : 1,
\ 'jinja' : 1,
\ 'html.handlebars' : 1,
}

[nionios]

This has been working as intended for at least 6 months