Red line across Paint Materials icon in Tools

In the lower left corner (the bottom of the Tools menu on the left) there are icons for Paint material which is a dimmed cube with a diagonal red line across the icon and another icon below it (mine is a white square). Tapping the material icon brings up the correct menu.

I just want to know why the Material icon seems to indicate something is disabled with its dimmed appearance and a red line through it???

It means that for the currently active tool - stroke painting = Off
You can activate it in the Brush Tool / Painting menu.

Sounds right, but the red line persists even after I close and open the project, and after I completely exit the app and restart it.

Maybe try to update the app to the latest version.
1.92 is live and 1.93 is on the way (minor releases)

That did it! I had 1.91 (and I had 82 other updates I forgot to do). Thanks.