Ranger "Refugees" - [5th campaign, 1st mission]

After reading countless posts and struggling with this issue (Biara), I concluded that it's a chronic bug in the game (no matter what patch is applied). After restarting the game from the beginning, Biara appeared in the 3rd week of the 2nd month, so I barely managed to complete all the "upgrade" quests and acquire the necessary spells. I even had to give up a town for this, but luckily, my army was already decent, and Biara had also lost some of her troops at the border.
P.S. If you give up a town, you need to reclaim it within a week. Otherwise, it's game over.