Knowledge base
How do I stop Performance Max from cannibalizing my branded search?
Apply a brand exclusion (or a brand list) and compare branded-versus-non-branded conversion share before and after.
Google has added ways to exclude brand terms from Performance Max over time, generally through a brand list applied at the account or campaign level. Confirm your account is using one. Without it, the campaign can serve against searches for your own company name and claim credit for a customer who was already looking for you.
To check whether this is actually happening, look at the "search terms insights" report Google provides for Performance Max and see what share of matched queries look like your own brand name and close variants. A high share there, combined with no brand exclusion applied, is the specific pattern worth fixing.