Loosing all my Preview Render Files when Editing with overlays

I have to output my final Video with two overlays: A timecode overlay for review and a transparent overlay logo from the broadcast station. I put them on top and lock those tracks.

Now I need to render preview files and use those for final export to speed things up. But sometimes when I edit a tiny bit underneath those overlays sometimes only the edited bit turns "yellow", loosing all rendered previews and sometimes the whole timeline suddenly turn unrendered.I am NOT altering or moving the other clips even by any frame, so the rest of the timeline should stay exactly the same. Why does this happen and how can I prevent it?

Do I have to split the overlays to multiple sections so it doesnt "unrender" my whole timeline?