Hello
It's actually rendered by the TCPDF library which we are using to generate invoices in Membership Pro. In latest version 2.16.1, we upgraded that library to latest version, so please try to update to latest version, then try again, it might work
If not, maybe you can send us the png logo you are using so that we can try to see why it is not displayed properly and get it sorted
Regards,
Tuan