When creating new field name for the metadata, the colon doesn’t work. Squiz removes it completely, but the full stop works. It does work in the Friendly name though.
For example: instead of twitter:card I have to use twitter.card.
twitter.card will affect SEO things, but you should be able to work around it by hardcoding the metadata fields into a standard page and outputting the values manually. Have a look at the templates on the matrix site which give you an example of how to do that: https://matrix.squiz.net/resources/templates/system-templates
We are also plannning on adding support for customising the metadata field output completely per metadata field which will make this a lot easier.
@anon82462363 I agree - though this is site that has been cloned from a very old site, so I suspect this was something that wasn’t rectified and a legacy thing.