Is a woman considered to be in the state of nifas (postpartum bleeding) if her fetus dies in utero while she is 12 weeks pregnant, knowing that the age of the fetus, whose growth has ceased, is 42 days, and she has not seen any blood, but merely a red tissue when going to the bathroom, and she is awaiting the natural expulsion of the fetus? Does the ruling of nifas apply to her?

The ruling of post-natal bleeding (nifas (definition)) does not apply to you unless the fetus is delivered. The red clots you are seeing constitute irregular bleeding (istihadah (definition)) according to the majority of scholars, and menstruation (hayd) according to the Malikis, Shafi'is, and Al-Layth. Continue your prayers and acts of worship until the fetus is delivered, and you must perform ablution (wudu') for every prayer.