SysRQ+B (boot) not working but O (turning off) is

For some reason, my Linux machine will properly turn off when striking SysRq+O, but **won't** reboot as expected when doing SysRq+B. Both of these ones show up in the help message (with SysRq+H) and all my SysRq options are enabled:

$ cat /proc/sys/kernel/sysrq 
1

I use a 11-years-old machine (Intel Q9550 / ASUS P5E3, assembled by myself) that currently runs a Fedora 31 linux distribution, but the problems shows up at least since Fedora 25. I first suspected the custom kernels from this distribution to be the cause, but it seems to occur with self-compiled genuine kernels too. It also used to work in the past and with other distribs such as Knoppix, but I can't remember when it started to happen. Nothing happens nor shows up when I do SysRq+B, neither on console nor dmesg log. Every other option (in particular, R, E, I or S) seems to be OK, and to produce log trafic. Does anybody know what could cause the reboot operation to be disabled ? This leads my to a "secondary" question: Inside the kernel source, at rows 447 and 469 of drivers/tty/sysrq.c, I can see:

&sysrq_reboot_op,               /* b */

…

        /* o: This will often be registered as 'Off' at init time */
        NULL,                           /* o */

So rebooting (which not currently works for me) is supposed to be always defined by default while turning off (which does work) is set to NULL, then defined at init time. I couldn't find which is this actor, nor at what exact time this powering off option is enabled. Could you help me on this too ? Many thanks in advance for your help. EDIT : lastest kernel at write time is 5.5.1 but the code quoted above belongs to 5.4.15 and older, and I'm running the Redhat's 5.4.15-200.fc31.x86_64 package.