As per the Adobe forum post here (https://community.adobe.com/t5/indesign-discussions/font-not-embedding-into-snippet/m-p/13153510#M489639) , I've added a form/text field into a snippet, but when placed the font used is replaced by Myriad... the font I want is a custom TTF. Is there a recognised issue here?

If I place the *.idms file with that TextBox formfield to the same document where I exported it from all is working good. But if I do a new document and place it there the font family definition reverts to Minion Pro. The style's name will remain. E.g. Semibold if I do this with your supplied font Ford Antenna.

That's a bug! A bug of the import filter for IDMS files. Inspect the IDMS file with your text editor. All the necessary properties and values are there like with my sample:

<TextBox
Self="ue8"
FontStyle="Semibold"
AppliedFont="Ford Antenna"
ReadOnly="false"
Required="false"
PrintableInPdf="true"
HiddenUntilTriggered="false"
Multiline="true"
Password="false"
Scrollable="true"
FontSize="17"

etc.pp.