How to decrypt Island / Work profile and Parallel Apps user in TWRP?
Note: this solution is tested on Android 10 (OxygenOS 10.3.2) in OnePlus 6 with TWRP 3.4.0.3.
The idea for things to work smoothly in TWRP is that your main user must not have any lock screen security/credential for the purpose of taking backup in TWRP. This way, TWRP can automatically decrypt main user, work profile user and also the Parallel Apps user.
Follow these steps:

Setup a Screen Lock to PIN/Password/Pattern for your main user/profile, if you don't have it already.
In Settings → Security &amp; lock screen, enable/tick Use one lock for Work Profile. This means that neither Android system, nor your work profile apps would ask for additional credentials to be used.
Set Screen Lock for your main user/profile to None. That is, remove lock screen security.

Now, boot into TWRP. TWRP would automatically decrypt your main user account (user 0), your work profile account, and also the Parallel Apps user (user 999). If you have setup a secondary user account, you can easily decrypt it with the command:
twrp decrypt CREDENTIALS SECONDARY_USER_ID

Alternatively, in TWRP, go to Advanced → Decrypt User → select your secondary user → enter its credentials.