A few minutes into the mission, a harvester shows up at my base, followed by 30+ fully upgraded units. You start with 15k credits, only one mammoth and cannot build a landing zone....after my base is destroyed, i get a cutscene where the LC destroys the UCS AI base and the mission goal changes to destroying the LC base. But since my base was already destroyed, i automatically lose the mission...

What gives? I playing the GOG version which is patch 2.1.