I chose to boot from /mnt/nvme0n1p4, but fossapup is loading a file from /mnt/nvme0n1p3.
First time you booted this install on nvme0n1p4.
Did you do a shutdown or reboot and make a save for it?
When asked, made that drive the location to put the save on?
If you are going to have Fossapup64 9.5 installed on multiple drive partitions.
You may need to give the save an added specific name for the install to use it with.
That way if the boot process sees a save on a different drive location.
Knows that is not the correct name or it finds multiple saves.
It will pop up asking you which save to use.
This addition to the name needs to be done when you make the save.
That is an option when making the save.
What I think is going on, is Fossapup64 9.5 no matter what drive partition you are booting from, is set to look for the name of the save file.
fossapup64save.4fs
The first drive location it searches and it finds that specific named file, it will use it.
0n1p3 is the first drive (partition) it will look in.
Or you put in the specific boot loader menu entry for each install.
Specific save location info, so it only looks there for the save.