most of the brahmin tribes are practicing endogamy in their own tribe. so it is not a matter of astonishment that bhumihar brahmins are not marrying with other brahmin community so frequently. Bhumihar Brahmin name itself has come in existence in early 19th century. Babhan is their ancient name which is mere pali word for brahmin. you might be knowing in nepali brahmins are said to be bahun similarly in pali brahmins are said to be babhan. Babhan name clearly suggest that they are ancient brahmins of magadh. Babhan word is there in Ashokan inscription. Sunga and Kanva are Brahmin dynasty of magadha. they have defied the basic rule that brahmins can only be pujari (priest). Sugania babhan is a clan in bhumihar brahmins. So please erase all your confusions. Babhans are the royal brahmins of magadha just like mohyal of panjab, tyagi of western up, niyogi of andhra, anvil of gujrat, chitpavan of maharastra, and Nambudhiri of kerala. Bhumihar brahmins are the royal brahmins who are priest also in places like gaya hajaribagh patna and benaras. — Agrarian Relations in Colonial India, Saran District, 1793-1920 By Anand A. Yang; The tribes and castes of the North-western Provinces and Oudh by William Crooke; ) are reliable document because all the authors are either renowned historians or social activist. these books clearly tells about the association of bhumihar brahmin with kanyakubj brahmin and it also tells about the old name of this brahmin as babhan. bhumihar brahmin name is quite a new name to babhan community that gained popularity only after 19th century. Origin of some of the clans of bhumihar brahmin is also written in the book. All these books never asserted the mixed origin of babhans/bhumihar brahmins. Mixed origin theory is a fake propaganda spread by some people to spread hatred. Spreading a false tale or rumour is not at all sensible so 
Pandit Hazariprashad Dwivedy and Sree Kameshwar Ojha believes; Bhumihar is a short form of Bhumi-Agrahar Bhojee Brahmin. Brahmins, who were responsible for Agrahar collection. King Harsha of Kannauj appointed them for this purpose.