angular.jsonのserveのブロックにbaseHrefとdeployUrlを追加する
こんな感じで
(略) "serve": { "builder": "@angular-devkit/build-angular:dev-server", "options": { "browserTarget": "wrio:build", "baseHref": "/subdir/", "deployUrl": "/subdir/" }, "configurations": { "production": { "browserTarget": "wrio:build:production" } } }, (略)