I tried a Select In Field action setting search string to an expression based on = 'MixMatch ... and it seems to work! (Theres a few hundred fields to test in the data model so I'm still a ways from declaring total victory)
However, I'd still like to see if there is way to fix the issue using the API. If anyone has any ideas, please post.
Can you provide the end example of what you found to work using toggle select with user input? I am interested in allowing users to input a list of style numbers to select at one time, and this sounds like it could help.