We had issues with billing over a year ago, brought it up with Google Cloud's billing department, who, like in your experience, told us all is well (when we knew it wasn't).
In the end it wasn't worth our time and energy to push it further.
It did feel though that Billing support was disconnected from the inner workings of Google's Cloud platform.
I hope a dedicated Product Manager from Google Cloud is reading this, so they can read our feedback and add it to the list of improvements needed on Google Cloud and its appendages.
Nick