Thank you for your reply.
This is a fresh install of Postman 10.12.10 on macOS. Settings are all at defaults, and I’ve checked that Request Validation was already set to ON.
I did import the API spec, and selected the option “OpenAPI 2.0 with a Postman Collection” (i.e. it imported the swagger API and created a collection at the same time)
However, if validation only works with OpenAPI 3.0, that would explain it 
If that turns out to be the issue, I’d like to know if there’s any workaround, e.g. using the Pre-Request script hook.
In fact, even if you know of any web site which can validate an API request against the corresponding OpenAPI 2.0, that would be sufficient. It’s just that I’ve been having trouble locating such a service, and so I had hoped Postman would have this functionality built-in.
I found plenty of tools for validating JSON docs against JSON Schema, plenty of tools for validating the OpenAPI spec itself (i.e. the JSON file which defines the API). But finding a tool to validate the actual requests against the OpenAPI spec is what I’m struggling with!