Promo code messaging disappears

I have built a promo code and entered text in the TR_WEB_SOURCE_NO to be displayed on screen when the promo code is activated.

That message flashes on the screen briefly, but disappears.

What might make that happen? Is there anything that governs how long that message displays? (I've never run into this before.)

The promo code itself works - the discount is applied in the cart as usual.

The first screenshot (I was lucky enough to grab before the maroon block at the top disappeared) is what should be there; the second screenshot is after the message disappeared.

Thanks in advance for any guidance!