The grunt-contrib-handlebars dependency in package.json was updated to version 1.0.0 in ticket
DS-3315. This version of grunt-contrib-handlebars uses handlebars version 4.0.0 to compile the templates, which causes conflicts with the handlebars version 2.0.0 used by Mirage2
This issue can be resolved by going back to grunt-contrib-handlebars version 0.9.3. This version is compatible with handlebars version 2.0.0.
During the npm build this will cause the following peer dependency warning to be shown, but this is simply because the dependency on the grunt version was configured too strict in that version of grunt-contrib-handlebars.
It does work correctly:
npm WARN firstname.lastname@example.org requires a peer of grunt@~0.4.0 but none was installed