Performance Max optimizes toward the easiest conversions available — and nothing converts easier than someone already searching your brand name. Without brand exclusions, PMax claims those sales and reports them as wins.
The diagnosis is simple: compare total revenue before and after PMax launch, not campaign-attributed revenue. If PMax ROAS is high but blended growth is flat, cannibalization is the likely culprit.
Apply brand exclusions, feed the campaign your customer lists as signals, and judge it on new-customer revenue. PMax is genuinely powerful once it's forced to hunt instead of scavenge.