System logs, for example, are compressed and then automatically deleted after a week. However, System Diagnostic Reports (whether it be a stall, spin, hang, crash, or panic report) shouldn't be automatically deleted. If you reset the NVRAM immediately following a kernel panic, however, the report is deleted (the NVRAM temporarily stores the report until reboot; if it's reset, the contents are erased). Short of forcibly removing the files (using Onyx, a similar app, or manually), this is the only situation I can come up with off hand as to why a report would vanish.

As a makeshift solution, however, you can save the kernel panic reports as a separate file.

1. From Console's menu bar, View > Show Log List
2. Select the panic report (kernel_[longstring].panic) from the "System Diagnostic Reports" drop down
3. File > Save a Copy As... > save file to folder of your choice

UPDATE:

Onyx is the most likely culprit, however, the option for clearing logs/reports is located in multiple places:

• Cleaning > Logs > System Diagnostic Reports
• Automation > Cleaning > Logs, including Diagnostic Reports

Uncheck these options if they are checked.

This can only be done on a mac:

• Connect your device to a Mac

• Open Xcode, go to Window->Devices in the title bar

• Click on your device from the sidebar

• Click 'View Device Logs'

You don't need to be enrolled in a developer program to be able to do this. To this on windows you'll probably need some sort of virtual machine/hackintosh