I am sure if it is related to .NET8. It could be related to IIS's memory management. Try considering below situations:
I am not sure if your application is running with single or with multiple instances. If multiple, it is possible that, one instance has created cache and other instance is trying to access the same.
Another possibility, IIS's memory management. If application is running on low memory, in such cases IIS flushes memory/chache by recycling app pools.