I downloaded the free Rookie parent theme and have no problem displaying translations of “Search Results for: %s” on a successful search results screen.
This works in the WordPress languages folder at wp-content/languages/themes/rookie-de_DE.mo as well as Loco Translate’s custom directory. It works there if the files are named correctly because the parent theme correctly loads the “rookie” text domain using “load_theme_textdomain”.
Are your files named correctly under the WordPress languages directory? Unlike the de_DE.mo file inside the theme folder, the system location requires the prefix so files should be named rookie-de_DE.mo