Could you please show screenshots of the copy control settings from delivery to billing for the delivery and billing type that you have used for the process?
Just to make sure that we are on the same page - this is not a proforma billing type (VOFA setting for SD document category is not 'U') - is this correct?
Have you tried to run program RVDELSTA in simulation mode? Does it show any changes that can be made?
Is the problem reproduceable or it happens sporadically? If it cannot be reproduced, check for ST22, SM21 and SM13 entries around the time the billing was performed.
Have you checked for custom coding in a user exit that could cause such behaviour?
Is the billing performed manually (VF01/VF04) or you use DSD with route settlement for example and billing is set to happen automatically with FSR?