Just on a whim my best guess is that the encrypted partition(s) is automatically mounted via /etc/fstab.

Assuming it IS NOT mounted to /, you could probably remove the line that mounts it on startup. Annoyingly, you’d need to manually mount the filesystem if you want to be prompted for the password.

I do this with a HDD that contains all the data shared on a NAS. It’s great because my data is still protected should it shut down for any reason, but less convenient for obvious reasons.