On Linux this is adressed by systemd-homed, which encrypts at least your home partition in sleep mode. Attackers could still try to manipulate the rootfs & hope the user doesn't detect it before using the device again.
"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 most valuable information for an adversary is typically found in Ram. Like your password manager master password, browser cookies, etc. Ram can be dumped easily with the right equipment.
The only safe encryption is on a powered down device.
If you fully hibernate to disk where it encrypts the memory snapshot to your FDE key, then you are good to go but that is not locking that is turning the computer off.
As long as that secondary disk uses a different FDE key and you manually unmount it. This is easily done with LUKS on Linux but YMMV on other operating systems
