How do you clear SSL leaf certificates mappings to particular domains
There's no caching of SSL-certificates like this. It is not a caching issue on the client.

The problem here is with the server - it presents the wrong certificate (from your point of view).

The reason that some clients see one certificate and others see another is most probably due to CDN and/or load-balancing, which means that you're not really hitting the same endpoint. It sounds likely that the server is using persistent connections (aka sticky sessions) which means that your MacBook sees one thing, but your iPhone and Windows 10 machine another thing.

This is something you need to solve at the server level. It hasn't got anything to do with your Mac as such.