The issue is that Roman Numeral capitalization is done last in the sequencing. There are pros and cons for various sequencing of the algorithms applied when modifying case. There are six of them. The sequencing is described in 'Alphabetic Case Transformations'.

'mix' unfortunately is a valid Roman Numeral. There is no easy fix for this. In the next release or patch I will introduce the ability to simply ignore 'mix' when applying Roman Numeral capitalization. When tagging the probability of a word 'mix' not being a Roman Numeral is pretty high.

For now a two step process can fix the issue. In the first step do whatever capitalization you 'really' want. This will possibly mess up the word 'mix'. Create an exception for 'mix' which has a case function of Capitalize. (Exact Match should be empty). Do a second Case function (UI or action statement) which uses 'No Change' as the capitalize function. Deselect all options except for 'Do General Exceptions'. In actions the statements could appear as:

Case Title Case in the Artist field with Roman Numerals
Case No Change in the Artist field with exceptions

The options specified on the first statement can be whatever you wish. The second statement should not be modified. I've attached a screenshot of the exception.