In the new modelling experience, when expanding the formula bar to display the whole formula in the new formula pane at the bottom of the screen it goes straight into 'edit' mode. This means that if you then click on another line item it adds '+[Line Item Name] onto the formula without you meaning to do so.
Anaplan need to change this so that a model builder can expand the formula, see it in the new pan but not automatically make it editable - this is so that you can click through and review the full formulas without risk of changing them without knowing.