GreedyMonk, in your case, one of the computers was just "lucky." By a happy coincidence, it found an alliance, most likely in the Warrior's Tomb. The second time, when you started over, such a coincidence naturally did not occur. To avoid such coincidences in the future, you can disable "undesirable" artifacts in the map editor, in the map properties.

Added 4 minutes later

I'm new to HOMM, so for now I'm just having fun playing on difficulty 200 with the "infinite resources" cheat. I'm playing the "Complete Collection" scenarios without mods. On the "Warlords" map, I came across something strange. One of the enemies (always a different one) manages to level up all skills very quickly in a short time and then, when we meet, defeats me completely, despite my cheat.

In general, for beginners, I would recommend playing on a lower difficulty, but without any cheats.