It might be worth specifically testing for yeasts if you still have the sample. Also I'm not so sure that's a low count for a blown pack, at least without asking the lab. They wouldn't have been expecting a count that high I assume and so would not have heavily diluted? But I'm not a micro expert.
It would be really helpful to your RCA to understand what it was that has caused your pack to blow. If it's something thermoduric as Tony has suggested, that points to a different issue than if it's something that should have been killed by your heat treatment.
Also I really would see if you can get the traces from the heat process. It will at least show you if there was anything else happening, any temperature drop etc, even if you have failsafes for it.
I'd basically work through the RCA step by step and get evidence that each was not an issue. So things like:
Set up and initial disinfection / sterilisation processes
Heat process
Any diverts if they occurred
Steam supply to the line
Disinfection processes for packaging, no shadowing
Sealing
Any evidence that you can find that there was no intervention into any sterile zones of the machine
Environmental testing
Product testing from retained samples
I'm sure you can think of more.