This is something which is very mysterious: How is it that a person’s countenance appears yet he himself is unaware of it? For certainly Yaakov did not know. Moreover, the value of the test, even though [Yosef] saw the countenance, is something which is mysterious and hidden from man.
דְּמוּת דְּיוֹקְנוֹ שֶׁל אָבִיו רָאָה (סוטה לו: תנחומא פ' וישב):
He saw D’mut D’yukno (the countenance) of his father. (Sotah 36b)
הוּא דָּבָר נִסְתָּר מְאֹד, אֵיךְ הַדְּמוּת נִתְרָאֶה, וְהוּא בְּעַצְמוֹ אֵינוֹ יוֹדֵעַ, כִּי בְּוַדַּאי יַעֲקֹב לֹא יָדַע. וּמַעֲלַת הַנִּסָּיוֹן, אַף שֶׁרָאָה דְּמוּת דְּיוֹקָנוֹ, הֵם דְּבָרִים נִסְתָּרִים וּמְכֻסִּים מִבְּנֵי אָדָם.
This is something which is very mysterious: How is it that a person’s countenance appears yet he himself is unaware of it? For certainly Yaakov did not know. Moreover, the value of the test, even though [Yosef] saw the countenance, is something which is mysterious and hidden from man.