Not much is known about Beonna / Beorna of East Anglia. There are a number of non-contemporary accounts of his 8th century reign, e.g., Historia Regnum (Symeon of Durham, Byrhtferth of Ramsey) 10th century, Chronicon ex chronicis (John of Worcester) 12th century, The Flowers of History (Matthew Paris) 13th century. Much of the written records of East Anglia are believed to have been lost with the destruction of its monasteries and by Viking raids and settlement.
A particularly interesting hypothesis posed by Marion Archibald and Valerie Fenwick proposed that Beonna and Beorned were one-in-the-same. They suggested that Æthelwald of Mercia installed Beornred to jointly rule East Anglia with Alberht. Following Æthelwald’s murder in 757, Beornred became king of Mercia but was soon disposed by Offa and forced to retreat into East Anglia once more. Beornred returned to East Anglia with an army and deposed Alberht. Beornred was in turn removed (again) by Offa when he asserted authority over East Anglia ca. 760-5. This is one of many hypotheses, of course.