Complex conditional formatting stopped working

[Kimber]

Up until today, I've been using a field to conditionally format the callouts on card visuals. All of a sudden, although the formatting is still being applied correctly, I can't see the name of the field in the selection box. When I select the field from the dropdown, the dropdown goes blank immediately again. Even worse, I can't apply this field to conditionally format NEW visuals. 

I haven't changed the definition of the field:

3rd Grade STAR On Track_Trend Color =

SWITCH (

    TRUE()

    ,'DataView STAR_StateReadingEnglish_Current'[Change in % 3rd Grade On Level as number] >= 'Style Elements'[Threshold Up]

        ,  'Style Elements'[Positive Color]

    ,'DataView STAR_StateReadingEnglish_Current'[Change in % 3rd Grade On Level as number] > 'Style Elements'[Threshold Static Upper Rng]

        , 'Style Elements'[Trending Positive Color]          

    ,'DataView STAR_StateReadingEnglish_Current'[Change in % 3rd Grade On Level as number] >= 'Style Elements'[Threshold Static Lower Rng] && 'DataView STAR_StateReadingEnglish_Current'[Change in % 3rd Grade On Level as number] <= 'Style Elements'[Threshold Static Upper Rng]

        , 'Style Elements'[Neutral Color]  

    ,'DataView STAR_StateReadingEnglish_Current'[Change in % 3rd Grade On Level as number] > 'Style Elements'[Threshold  Down]

        , 'Style Elements'[Trending Negative Color]

    ,'DataView STAR_StateReadingEnglish_Current'[Change in % 3rd Grade On Level as number] <= 'Style Elements'[Threshold  Down]  

        , 'Style Elements'[Negative Color]

    ,"#000000"

)

I've verified that the field returns a valid Hex color: 

I've also verified that all of the other fields referenced in my 3rd Grade STAR On Track_Trend Color return valid data.

Does anyone have any idea what could be going on here?

[Kimber]

I've narrowed it down. The problem is introduced when I reference 'Style Elements'[Positive Color] (or any of the other color fields). Here's the definition of [Positive Color]: 

If I paste the hex value into the formatting field, it functions as expected. It's just the reference that makes it stop being able to be selected.