@darkdozy, I have not managed to reproduce this issue. I have successfully completed that showcase about a dozen times with different outcomes on versions 1.66.139586 (main) and 1.66.139792 (profiling).

Try restarting your computer and then verifying the integrity of the game cache for Arma 3.

1. Restart your computer and launch Steam
2. From the Library section, right-click on the game and select Properties from the menu.
3. Select the Local files tab and click the Verify integrity of game cache... button.
4. Steam will verify the game's files - this process may take several minutes.

Then restart the showcase and try again.

If that does not help then could you use the Arma 3 Launcher to prepare a troubleshooting report and then upload the report to the feedback tracker for the developers to review?

For issues regarding Arma 3 you can file a troubleshooting report, also via an action available in the option menu in the upper right corner (gear icon)

Be sure to upload the whole report file. If the report file will not upload directly to the feedback tracker then try uploading to a free file sharing service such as Dropbox, Microsoft OneDrive, Google Drive, Mega.nz, and so forth, and then link to the report file.

I managed to reproduce the issue after verifying game files and restarting showcase on dev build 1.67.139899 x64 version. Mission seems to be stuck on the optional objective with the orange smock which I did not complete. I ll try again see if a manage to finish it.

I restarted the mission and was able to finish it several times.