Key is in memory at all times after boot on all of those.
Full disk encryption is only useful on a laptop if the device is powered down fully.
Full disk encryption is only useful on a laptop if the device is powered down fully.
It is hard to fix this too, because almost no background desktop processes behave well when they are suddenly unable to write to the disk.
Even if you solved that, your password manager has keys in memory, your browser has cookies in memory, etc etc.
It doesn't seem unsolvable, as long as sleep (closing lid) suspends all activity.
(lock with background activity is different, lets discuss the sleep case)
"If your Mac has the T2 Security Chip (recent Intel-based Macs) or uses an Apple silicon chip (M1 family and future), security is significantly improved. The Secure Enclave in both systems uses encrypted memory, and has exclusive control over FileVault keys. The Intel CPU or the Application processor (M1) never sees the keys, and they are never stored in regular (unencrypted) RAM. Due to this, an attacker would only be able to extract encrypted keys (which can't be decrypted), and only if the system failed to prevent the DMA attack in the first place. These protections make it a lot safer to leave your Mac asleep."
The only safe encryption is on a powered down device.
I would think it would have to be while the device is mounted and OS locked, but surely if you dismount a secondary disk/container the key is purged?