Even if a user from XyzReviewers group approves this behaves like not, in a conditional merge check script.
import com.atlassian.sal.api.component.ComponentLocator
import com.atlassian.bitbucket.user.UserService
def userService = ComponentLocator.getComponent(UserService)
!mergeRequest.pullRequest.reviewers.findAll { it.approved }.any {
userService.isUserInGroup(it.user, "XyzReviewers ")
}