There are no lockdown mode restrictions on my system. Kernel is not signed, but i switched to UKIs a couple months ago (hibernation worked fine with these).

It is worth noting that the failure is hibernating, not resuming. Normally hibernate takes ~1 minute with fans spinning at max speed, but it recently started not finishing and instead being stuck on a black screen for more than 30 minutes without the fans running until i run out of patience (i hibernate before i go to sleep or head out) and force power off the system (power button 10 second press on my system).

it worked fine before, so why break now?

5gb ram usage of 64gb total & 0gb used on a 128gb swap partition is quite unlikely to run out of swap space during hibernate

My secure boot with hibernate works perfectly fine, or rather it did work fine before hibernate started freezing my system, secure boot or not.