Text styles have a hierarchy similar to messages. You probably know that, but for others:
The style of the mainstack
The style of a substack(s) if any
The style of a field
The style of the text, or a selection of the text, in the field
From the bottom up (I think that's your preferred perspective) each one overrides the one above it. Your original post:
The inspector sets the style of the entire field, not the characters within it. If the text characters were bold, then changing the field style won't change its appearance because the character style overrides the field style.All of a sudden if I try, in the inspector, to set the textStyle of a field that is already bold to, say, plain, the box blinks at me for half a second and then reverts to bold.
To set the style of the text itself from the inspector, go to the contents pane, select part or all of the text, and use the main IDE menu to set a style. That will override the field style.
I don't think preferences have anything to do with it. More likely one of your message box commands changed it. If you script something like "set the textstyle of line 1 of fld 1 to bold" then you are dealing with character styles, not field styles.