Carlo, I think the Theme style formatting is overruling the formatting of your new field.
I have attached you sample again as images, but now with 2 extra instances from your field.
Both were created from the menu, then the formatting was copy / pasted via the inspector and conditional formatting was applied.
Was the first field created in FMP11, then converted to 12?
(apparently i cannot upload a file in a reply, only images, hence the pictures)
I forgot to mention that all these fields are globals and were originally (with others that are behaving badly) converted from fm11, the theme is still classic.
I applied the new formatting (gradients) in FM12 as well as the (null) conditional formatting on the background. I am also experiencing the same problems when copied to other layouts (even after I have reapplied the conditional formatting) with the result being random (i,e some disappear and others don't) . I feel it is very much like copying of variables from one layout to another in FM11 and you had to click on the variable to get it to appear.
The Lesson is you can not rely on the behavior of conditional formatting from one layout to another when copying global's I am not certain about other fields but the file is mocked (attached) up from my original and attached was a text field. This is going to be a real pain having to check on all conditional formatting it may also be linked to the issue of FM12 being so slow when you have multiple conditional formats on a layout.