A grant ID without a publisher prefix should trigger a warning, which could be a Validity - 'something doesn't look right' type error.
Our normative recommendation in Standard is that all grant IDs are prefixed by 360G-XYZ in order to ensure that a grant ID is globally unique, once published.
There are some publishers using their funding org ID as a prefix instead of 360G and although this isn't recommended it still does make their grant IDs globally unique.
I hope it would be simple to add a check with a RegEx to look for any grant ID in the 'identifier' (json = id) field that didn't start with 360G.