Hello, I have had this trouble ever since allowing ubuntu-derived and debian/devuan to coexist on the same drive. I often lose the uefi boot entries except for one that is labeled 'neon' or 'ubuntu' or just the hard drive's name and windows (internal disk) in the uefi boot menu, and must use uefi boot to add it back, or, until recently, use grub from whichever neon entry I used (anywhere from one to three show). Until last night (when I got stuck on a loop of the the acer logo flashing before rebooting automatically again), I have been able to just enter from the neon grub (regardless of which uefi entry lead to it). While I have shim installed, now trying to boot from neon's grub (so I can use efibootmgr in devuan proper) shows that the shim signature is bad and that the kernel needs to load first. I will try to reboot into devuan as soon as the kde partition manager finishes canceling (It froze at 25%, so after an hour hit cancel) or I surrender my ebooks which I have copies of.

More details:
From within the grub menu, I can run ls, and everything shows up. My typical hard drive is hd0 if no live media is used, and otherwise it is hd1 (with the live boot media being hd0), and the internal drive (which I cannot access or see once booted) is one index higher, and still hd (although it is an ssd). Prior to the latest trouble, I went 85 days without rebooting, then rebooted twice in one day (I doubt this is the issue). I am using an Acer Aspire 515, and my shim versions are below:
shim-helpers-amd64-signed is version 1+15.8+1~deb12u1
shim-signed:amd64 is version 1.44~1+deb12u1+15.8-1~deb12u1
shim-signed-common is version 1.44~1+deb12u1+15.8-1~deb12u1
shim-unsigned:amd64 is version 15.8-1~deb12u1

I copied /etc/os-release to /etc/lsb-release since that was suggested on an arch forum thread, but firmware was a potential issue too, although I haven't messed with firmware lately, and have intel-microcode installed along with firmware-iwlwifi neon only has the additional amd64-microcode, firmware-sof-signed, and iucode-tool, which I am working on installing through chroot now.

Update: installing those did not help, and now I cannot even enter into uefi settings or uefi boot menu; that is just a frozen tty. mokutil shows secure boot is enabled again, as booting into neon sometimes does. But I have the current workaround of mokutil to disable secure boot and efibootmgr to recreate the entry and make it be nextboot.