premiere randomly deletes prerenders

while working i encountered a strange thing.

i have a some split screen animations and graphics for frames and titles in my timeline, so i have to render certain parts to see them. 

Now, when working at the end of the timeline and move parts (up and down the tracks or a few frames let/right)  it always happens that already rendered parts much earlier in the timeline randomly turn red again - while not affected at all. 

anyone knows how to avoid that?