My only question is this:
Are there any signs that the brass has expanded just ahead of the belt?
If there are no signs, such as a shiny mark where the brass has been 'wiped', then there is no reason to assume that this is the problem.
Do you neck or partial FL (read shoulder bump .002") size each time?
How many times have you trimmed this brass?
Do the cases show any 'wiping' on ANY area?
The reason I ask these questions is that I can get in excess of 20 firings from my comp rifle in 300WM.
I don't run them at full pressure and neck size only until they become hard to chamber, then bump the shoulder .002". I also anneal after every second firing.
On my current batch, I have only trimmed them twice and they have had 22 firings/sizings to date and are still going strong.
The 'bulge' in front of the belt is the normal expansion ring, as found on all cartridges and only becomes a problem to those that FL size every time due to thinning/stretching in this area. A tight chamber and neck sizing almost eliminates this 'phenomenon'. Case length growth is also slowed by neck sizing only, and only partial FL sizing when absolutely necessary.
If you find that your cases have a bulge above the belt, you can FL size back to SAAMI specs and start all over again, but you will have to check that you don't have incipient case head seperations after the initial firing forming again back to chamber specs.
Running a bent paper clip inside the case can 'tell' if there is an expansion/thinning line developing.