Question:

Usually in the winter season people wear warm hats on their heads by which their foreheads are covered. Is it necessary to uncover the forehead for Salah? If one performs salah without uncovering their forehead, will the salah be valid?

Answer:

To perform Salah in this manner is makrooh tanzihi. However, if someone did such due to severe cold conditions, then there will be no problem.

ويكره السجود على كور عمامته من غير ضرورة حر وبرد الخ (مراقي الفلاح) الظاهر أن الكراهة تنزيهية. (طحطاوي على مراقي الفلاح 195) كما يكره تنزيها بكور عمامته الا بعذر. (در مختار)

And Allah Ta’ala knows best.

(Hazrat Mufti) Ahmed Khanpuri (May Allah preserve him)
The answer is correct. (Mufti) Abbas Dawud Bismillah (May Allah preserve him)
Mahmoodul Fatawa Vol.65 Pg.223-4