I have observed a similar thing, booting from an intel optane. u-boot loads the kernel from NVMe, but during linux kernel init the board reboots. had the situation were this was going on for quite some time, but after a few power down and power up cycles, i usually only get one reboot and then the kernel loads fine. Looks almost like some consumption peeks during the initial boot which are not there / not so bad during the second boot attempt ¯\_(ツ)_/¯. Haven’t found a specific point in the kernel boot logs were the system resets. By now it ‘kinda works’ as in: i’m powering the board on and its boots quite consistent, with maybe one quick reboot on the way.
Using a patched u-boot (for 2x2 pcie3) from here How to build u-boot spi images