The null result was caused by the bluebird promise library, which shims promises globally and breaks the google-cloud client internals. Removing the global bluebird shim fixed it and accessSecretVersion started returning the secret payload. If you see null results from google-cloud Node clients, check for promise-shimming libraries first.
Source: https://github.com/googleapis/nodejs-secret-manager/issues/33
Source: https://github.com/googleapis/nodejs-secret-manager/issues/33