Yo! Dudes,
The only reason I know of is when the 3D Wireframe mode is activated or even Hidden mode.
View - Shade - 3D Wireframe / Hidden.
When you make a mesh (or solid objects) you obviously want to use hide or gouraud to see what you've made.
If you hit 2D Wireframe the text will revert back to it's solid appearance.
This is for text that is in model space of course.
Then again if it is text in paper space and you activate the 'ordinary' Hide command........View - Hide, you will get the same effect, until you clear this effect by hitting Regen.
I came across this years ago (R12) & found out by accident how to clear it during one of the C&G AutoCAD computer classes.
As to why this should happen with text.........I dunno!
It's a programme thing I suppose.
regards
Stephen