Ok ... I replaced the shockwave_alpha.png and now the models are not disappearing any more. Really great ... thanks again.

Unfortunately it's not fixing the TargetMap issue. I hope it's not a bug in my project that overwrites the render_target pointer in some way.
But then again the render_target should be updated with every frame, shouldn't it. So I guess it would lead into an engine crash if something would write into its address.

The render target pointer should change for every view that renders into a render target. Maybe you are using some other shaders with multiple view stages that use render targets like shadow mapping.

Your problem is that your material doesn't belong to the second stage after the camera view. There is no sequential order. When your material is processed, the render target pointer could point to another/changed render target of another view that was calculated in the meantime.

I think you already did the right thing by rendering into a seperate bitmap and using this bitmap in your shader instead of TargetMap.

You can also draw the render_target bitmap onto the screen with help of the Debug_Bmap macro. Maybe do this and remove other views and shaders and check if its rendered content is correct.

Actually there's no other view, but I will give Debug_Bmap a try tommorow.
I'm really happy that everything is working now inside my project ... though I would still like to know why I can't use the render_target bitmap.

Maybe Ezzett is right that the reason is that the material is not assigned to the view.