From the uMatrix wiki:
The logic behind referrer spoofing is simpler now: it’s whether the switch referrer spoofing is turned on, and whether the domain of the referrer URL is third-party to the domain of the request URL. Whether the domain of the URL of a request is whitelisted is now irrelevant.
I understand that you prefer to use the whitelist for requests from a domain instead of requests to a domain (like RefControl does), so that it follows the logic of “scopes” applied by the other uMatrix options.
However, have you considered whitelisting requests for both cases: whether the request happens within a whitelisted scope or to a whitelisted scope/domain?
For example, this page and many other websites use Solvemedia captchas, which require a valid referrer. Instead of whitelisting all those websites manually, it would better if whitelisting Solvemedia domains was enough:
referrer-spoof: api.solvemedia.com false
referrer-spoof: solvemedia.com false