As I said earlier, Fast Startup is just a special variant of system hibernation.
In normal hibernation, both the OS state of Windows itself and the user(s) state(s) of those logged in at the time of hibernation are written out to disk. These are retrieved when the system is next powered up.
In Fast Startup, only the OS state of Windows itself is written out to disk. It is, of course, retrieved on power up. The reason this can become problematic is if someone does not restart their system every once in a while, and uses Shutdown almost exclusively, corruption can creep in to that special OS state hibernation file, and if it does you can get some very, very bizarre behaviors that are almost impossible to fathom unless you know about this possibility and try doing a Restart. Restart forces a complete shut down, with absolutely nothing about the OS state saved to disc, and when the machine comes up again it loads Windows 10 from scratch from disc.
If hibernation is disabled, it's disabled. It's an all or nothing affair, and since Fast Startup is a special case hibernation, it is automatically disabled when hibernation is disabled.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Tolerance is the positive and cordial effort to understand another's beliefs, practices, and habits without necessarily sharing or accepting them.
~ Joshua Liebman