On Linux, restart_app replaces the current process through execv. By default, the restarted process launches without the original command-line arguments. To preserve them, call restart_app_plugin_store_argv in your linux/main.cc before running the Flutter engine: 
#include <restart_app/restart_app_plugin.h>

int main(int argc, char** argv) {
  restart_app_plugin_store_argv(argc, argv);
  // ... rest of main()
}
Most Flutter apps do not rely on command-line arguments, so this step is optional.