Thanks Razazel.
I managed to avoid this bug with your help. Below are the steps I took:
- As 'Resuming' led to the bug described above, I reverted to the previous 'Gori' save point - created after completing the previous mission.
- I tried to go on a patrol using the quadbike to see if Arma 3 would let me. I had the opportunity to select a location to start the patrol from. However, the first time round, I was sent back to Arma 3's main menu after the loading was complete.
- I restarted steps 1 and 2, picking up a different patrol start location. This time, the patrol kicked in without problem.
- I returned to base, which automatically created another save point.
- I tried launching the Breaking Even mission again... with success!
I guess I'm happy for this issue to be closed.
Thanks again for your help.