To be clear, please don't conflate reading and reciting the Vedas to having understood the Vedas.
I didn't come across anywhere in KMG Mahabharata where Karna is said to have read/recited vedas. At most he was fully conversant with the vedas and understood the eternal saying of the vedas (Udyog Parva)
Maharishi Matanga, who was born a Chandala, because of his penance got the boon of God like status. But I could not find any reference of him teaching vedas to any disciple
Jabala was son of a prostitute. and his father was unknown. For all we know he is from a Anuloma marriage. So his example would not count
Romaharṣaṇa , a Suta, who was killed by Balarama had obtained Brahmin like status. Thus Balarama was exiled for 12 years in accordance with Manusmriti. However there is no mention of Romaharsana teaching the Vedas. He was killed when reciting the Puranas to other Brahmins.
And to emphasize, I am not looking for varna migration or getting into guna vs birth based varna classification. I am specifically looking for examples and/or statements in scriptures that categorically show Pratiloma or Shudra born have read/recited/taught Vedas or were allowed to do so
EDIT : Seems that even seasoned users are getting confused between pratiloma and anuloma born. Without getting into details, anuloma born are those where father is of higher caste than mother. Pratiloma born are those where mother is of higher caste than father