I've exported a few WebKit changes to the WPT repository using the appropriate export tool (Tools/Scripts/export-w3c-test-changes) and the result has been that wpt-pr-bot leaves the "This patch has been exported from WebKit; it will be approved automatically once the downstream patch is r+." comment but is not added as a reviewer leaving the PR with no assigned reviewer. Here's the most recent example: web-platform-tests/wpt#50756.