Timeline: rendered effects don't stay rendered

I've noticed many times that, after I "render effects in work area" or "render entire work area", the rendered clip does not remain rendered (ie, the bar above the clip on the timeline goes red again), even though I have not changed anything on that part of the timeline (no changes to the clip, no new transitions, etc). Eg, I have rendered effects on the entire timeline, so that no clips have the red bar, then save the project; I come back to the project later, and some of the clips will again have the red bar. Why does this occur? (Premiere 24.6.1)