The important bit here is that hibernation is both a software state and a hardware state.
When you press the Hibernate button in Windows it saves the contents of RAM to disk so that it can recover from a power loss. Then it tells the hardware to enter state "S4" which is a low power mode that isn't really off.
Your motherboard likely has states S0-S5 with S0 being the "ON" state and S5 being the "OFF" state. When it receives the command to enter S4 it doesn't really power off. On power up it is able to skip the BIOS and may also skip features that you want, like BitLocker.
What you are asking for
It sounds like you want to use the software Hibernation features and then put the Hardware in to S5, full power off. Then you want to do the whole boot sequence, including BitLocker, but then rapidly resume your Windows session.
- To do this you could physically cut power to the system after it enters the Hibernation state. You would do this by flipping the switch on the PSU for a desktop or pulling the battery on a laptop.
- Hypothetically, to do this automatically you would need the ability to shut off state S4 in your BIOS. This would need to be a feature on your motherboard. Go in there and look around. You want to disable S4 Hibernation or otherwise force mode S5 when S4 is called. That may or may not be a feature of your BIOS.
Disable hibernation with:
powercfg.exe -h off
This way your computer is either in sleep mode, with quick recovery. Or it is fully off. It will never again be in a state where it appears to be off but is actually unencrypted. You must use full shutdown every time you want to encrypt your data at rest.