As far as I remember, the event where you replace 300 angels with another unit was present in the classic version of Heroes. With HD, if I'm not mistaken, this replacement didn't happen! But I quickly figured out this "trick" and reloaded the save in a different folder.

The problem is specifically with HD+. This option, in addition to changing the terrain of battlefields, also affects other game aspects. With HD+ enabled, that hero died on the event with the Nagas. Fortunately, in Xiedu, this happens in front of the player, and the guardian of the quest requires a victory specifically over the mentioned hero, so it was obvious that something was wrong.

However, when playing Les prémices de l'Apocalypse, I was less lucky; the key hero died in the same way on an event, but I only realized this at the end of the playthrough. But even so, it's a rather confusing map, although not uninteresting.