Madhvacharya was a Dvaita philosopher who propounded Dvaita philosophy by writing commentaries on Prasthanatrayi. I heard that his appearance was foretold in Saura Purana as an incarnation of a demon named Mukhasura. Now, even Wikipedia article on Saura Purana also says chapter 38 to 40 of Saura Purana attacks Madhvacharya. Is this true?
Yes. Saura Purana chapters 38 to 40 contain prophecy of the birth of Madhvacharya and passages which attack Madhvacharya and his philosophy.
These chapters glorify Advaita and Shiva. There are frequent usage of the six darshanas viz Sankhya, Yoga, Nyaya, Vaisheshika, Mimamsa and Vedanta. There is even glorification of Advaita. Not only that, there is also a mention of non vedic schools like Charvaka, Bauddha, Mlechha and Jaina dharmas. There is also mention of Shankaracharya, Badarayana Sutras (perhaps Padma Pada also. I couldn't understand the verses which contain the name properly).
These chapters talk about the characteristics of Kali yuga and prophecies that there would be falling down of all the varnas and the situation would be terrible where Brahmins would be uncultured. In chapters 39 and 40, Suta says the back story of birth of Madhvacharya. According to the story, it is said there is an evil minded person named Madhu who was cursed by his Guru was born as Madhvacharya in southern coast of Karnataka (!).
The curse goes to a very specific detail. It mentions that Madhu will be born will write a commentary on Brahma Sutras. Interestingly there is exact mention of the name along with birth place. He would tour countries like Utkala, Maharashtra, West Bengal, banks of Ganga and Godavari preaching his philosophy. As the time in Kali yuga passes, the philosophy preached by him will get popular and the shishyas would be evil too who do Shiva Ninda.
Hence, the Wikipedia claim is correct which says Saura Purana's attack on Madhvacharya.
You can read Saura Purana which is a upa purana of Brahma Purana on wayback machine - Saura Purana ASS.
The Saura Purana texts don't talk about Sri Madhvacharya. The text talks about a certain Brahmin named "Madhu Sharma" who was born in Andhra Pradesh (Sri Madhvacharya was born near Udupi, Karnataka). Madhu Sharma is said to be the disciple of Padmapaada Acharya. (Sri Madhvacharya was born 400 years after Padmapaada Acharya). There's too many such historic discrepancies which clearly indicate that this text is not about Sri Madhvacharya. Moreover, there's too many grammatical errors for the text to be authentic. It's more likely that it's interpolated.
Someone has commented in the thread, " In chapters 39 and 40, Suta says the back story of birth of Madhvacharya. According to the story, it is said there is an evil minded person named Madhu who was cursed by his Guru was born as Madhvacharya in southern coast of Karnataka (!). "
This is untrue. Kindly go through the texts once again. Saura Purana Chapter 39 verse 39 states that a wicked minded Brahmin will be born in central Andhra Pradesh. And there's no mention of any southern coast of Karnataka.
तदा"न्ध्रीदेशमध्येतु" दाक्षिणात्यो भविष्यति | ब्राह्मणो दुर्भगः कश्चिद्विधवाब्रह्मणेतरः || ३९.३९ ||
For more info, please go through this rejoinder by Shri BNK Sharma
"AN ATTACK ON ŚRĪ MADHVĀCĀRYA IN THE SAURA PURĀṆA on JSTOR" https://www.jstor.org/stable/41688222?seq=1