It's not unusual to have sleep state issues on Linux, which is what it runs (an Arch derivative).

Could be some faulty hardware somewhere, but it's not a concerning enough issue to spend money replacing things for a machine that gets used for personal projects and video games.

With fast boot, it is a partial-hibernate (as you said, it is basically logging out then hibernating). But also, you are shutting the device down. You can hibernate a laptop and take the battery out. The device is off. Thats the difference between sleep and hibernate. Sleep still uses power.

Hibernate is a true power off, with instructions for the PC to load ram from disk on next boot. IME hibernate often works more reliably, because sleep states can be buggy, and hibernate just powers off the device. Hibernate is slower though, and can have its own issues (often related to disk encryption)

Sleep being buggy is a mystery to me, but has happened on nearly every laptop i've owned. I think too many things can wake slept device or prevent sleep. Leading to "I closed my laptop and put it in my bag. Then it woke up and got hot and span the fans until it died". But ive also had many instances where some apps are more buggy after waking from sleep (ive had issues like video playback failing after wake, presumably related to hardware video decode), or certain peripherals don't handle it well, and windows reshuffles monitors or changes a microphone since the "default" was briefly gone, or a mouse/keyboard doesnt wake the device from sleep when they should, etc. Phones and tablets seem to handle low power states really well, but ive literally never had a good experience with it on a laptop or desktop.