Can cerebral amebiasis be asymptomatic?

I would say no. The tests to verify this condition are invasive and I cannot see why anyone would test for it without symptoms. The central nervous system is not the natural habitat of this amoeba, but it is one where they would find ready sources of nutrients to feed on and replicate. If present that activity would cause symptoms & it would make itself known.

No such situation has ever been reported or documented. A brain infection always causes severe symptoms.