Have you checked against (not provided)? That may well have swallowed a large chunk of branded
BTW, the big (not provided) switch was early March for the UK and it makes a heck of an impact on reporting
Other things to check:
have you stopped any marketing activities in other channels that would have driven brand traffic?
Are your segments correct? Double check, maybe someone changed whats in them. How are you handling (not provided) with respect to assigning brand or non brand?
Very unlikely: tracking code misfiring on your homepage i.e. affects brand count but not longtail
Stephen