The engine Icon you are referring to is bright white when ICE is engaged and greys out or dims when running on Electric only. Very subtle, but there it is.
Here's something I noticed by chance on my 2025 Lariat. If you notice the engine icon in the center information screen, on the left side, the icon will brighten and dim according to drive status. ICE engine engaged; icon is bright. When running on electric only, the icon is dimmed. Very subtle...