I don't think it is B9 Software
The white area is the exposure that is solid red areas (not shown) would be not printed as well as any Black would not be printed.
Use the Xray feature in the B9 software to scroll through the design to see if there are any black areas in the middle of the model.
Did you use the model fix tool in matrix?
If there is any hollow areas it would fix them.
You can send me the file and I will look at it tomorrow if you firstname.lastname@example.org
Zip it and send it to me or send me a link to your cloud storage.
I will check it out