I refer to this document:

https://reference.opcfoundation.org/src/v104/Core/docs/Amendment4/readme.htm

In chapter "7.36.2.3 EncryptedSecret Format", this is written:

"The PayloadPaddingSize adjusted with the following formula:

    If (Secret.Length + PayloadPaddingSize < InputBlockSize) Then
            PayloadPaddingSize = PayloadPaddingSize + InputBlockSize

Where the InputBlockSize is specified by the SymmetricEncryptionAlgorithm."

The problem here is that InputBlockSize is not well defined for ChaCha20-Poly1305.
And some words about the rationale is why this extra padding is suggested, is also
needed. I think I brought this issue originally up in one of our security working group
meetings, and the reason for it was like this:

This avoids leaking any information about the actual password size in case it is
shorter than 16 characters, to make dictionary attacks somewhat less easy to
accomplish.

Since in the case of ChaCha20-Poly1305 the InputBlockSize can be an arbitrary
value, it is recommended to use 16, as in the case of AES ciphers, since it avoids
the possibility to leak information about the exact password length in case the
same user credential is encrypted according to different security policies.