Why rendered audio becomes unrendered after restarting premiere?

My project was residing on a USB drive dock. However, I shut off the dock and was still able to reproduce the problem as follows:

create a new project

create a bars and tone 

create new sequence and name it "parent" and put the bars and tones clip in it

create another new sequence and name it "grand parent" and put the parent sequence in it

At this point the audio wave can be seen in both sequence

open parent sequence and drop an audio effect on the bars and tones clip e.g. reverb

the waveform disappear from grand parent. Render audio in this sequence; the waveform appears

save the project, close the project, and re-open the project

the waveform in grand parent is gone and needs to be re-rendered

I don't know if this is related, in other projects sometimes when timeline is zoomed in a lot, the audio waveform disappears. If I zoom out the waveform re-appears.