Was my adopted storage decrypt key lost despite full nandroid backup in TWRP?

I have my phone encrypted and my sd card is used as adopted storage. I was about to change my /cache and /data partitions from ext4 to f2fs. Now I'm unable to decrypt my sd card. I use twrp 3.0.2-0.

What I did was:

1. Create a full backup of all available partitions on adopted storage. (using TWRP)
2. Copy the backup files to my PC over MTP.
3. Change /cache and /data partitions' file system from ext4 to f2fs.
4. Restore the backup of /cache and /data (first forgetting about the rm -rf option and thus having to repeat the previous step for /cache).
5. Boot up and notice I can't access my adopted storage any more.

It seems that my backup of /data partition does not include the /data/misc/vold/expand_$PARTGUID.key file. The /vold folder seems to not be included in the backup, while the other folders in /data/misc are. Should I have known this?

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