The "TextStyleAdd()" function, like all calculation functions, affect the value stored in the field. Thus it will affect all presentations of that stored value.
If you want the presentation to differ between layouts, use "TextStyleRemove()" on the stored value. And apply formatting to the layouts instead.
With best regards Magnus Fransson.
+1 to fran55on.
When you apply ANY text formatting function changes the data at the data layer.
You want to affect the presentation layer only so remove the text formatting function that is changing the data and leverage themes/styles and the layout object presentation attributes in the inspector.