When using a conditional formatting rule to set the background color for a Multi-line text control, the background color is not changed the way that it is with a Single-line text control with the same rule.

Adjust the padding of the Multi-line text control through the control configuration to add a slight border to the control.

The difference in behavior between the Single-line text control and the Multi-line text control is due to the size of the "text" area within each control type. The Single-line text control border stretches beyond the "text" area, and already gives the control a slight border. The Multi-line text control has no outline as the "text" area of this control stretches to the edges of the control bounds. 

The formatting rule actually applies the color change to the background of the controls to which it is applied, not the "text" area itself.