How to redirected output from disowned process to a file

A borgmatic backup command that runs for many hours: long_running_cmd &> file.txt I did Strg+Z then bg then disown to keep the command running I case my laptop goes to sleep or disconnects. I thought the whole command chain just gets executed in the background, but I notice it did not write into it any more. How do I ensure that the output of a command is written to the file even after I disconnect my ssh connection? (*if possible with default commands that can be found in a Debian headless server installation*)