Unfortunately this seems to be a trait of many acacias. they are fast growing pioneer plants but most are also relatively short lived and prone to branches dying. Most of the species I have tried have similar dieback regularly.
I would not worry about the dead section. Plants are able to shut off sections to prevent problems from moving through the entire plant. It is highly likely the dead section will stop at the nearest fork. Cut it out if you wish but leaving it should not threaten the whole tree.