This bug appears to still be present in version 6.6-1, the current in buster. Additionally, Jawaad's proposed patch (no longer?) appears to work, as it provides the multiple recipients space-delimited, where mail wants them to be comma-delimited, at least for me. In smartd-runner, I changed:
recipients="${@}" to: IFS="," recipients="$*" which appears to work, but I won't claim this is perfect. Maybe the 10mail script could be changed instead? Happy to help develop or test an acceptable solution and get this bug closed after eight years.