Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #14

وقد كان من قصة يوسف بن يعقوب عليه السلام ما جاء به سورة في القرآن وتضمنت استتار خبره عن أبيه وهو نبي الله يأتيه الوحي صباحا ومساء وما يخفى عليه خبر ولده وعن ولده أيضا حتى أنهم كانوا يدخلون عليه ويعاملونه ولا يعرفونه وحتى مضت على ذلك السنون والأزمان ثم كشف الله أمره وظهر خبره وجمع بينه وبين أبيه وإخوته،وإن لم يكن ذلك في عادتنا اليوم ولا سمعنا بمثله.

There is the story of Yusuf, the son of Yaqub, to which a Surah of the Quran is dedicated and it recounts how he was hidden from his father, an apostle who received revelations day and night, yet the news of his son was hidden from him and his sons, who would meet him and transact with him, but do not recognize him. Years passed like that before Allah revealed his story and united him with his father and brothers. Such an event is extraordinary and we have not heard the like of it.