I am still having this problem as well.
Here is a video of it happening: Upload files for free - 1small.mov - ufile.io
In this case it shows an existing annotation, go to the search menu, re-open the pdf, the annotation is gone.
I have also tried other android tablets: a chromebook (not an android tablet, but I installed the android app), and an Onyx ebook reader. On these the problem -seems- to happen much less, but annotations still disappear (and sometimes reappear).
Unfortunately I believe that Samsung is the most popular brand of android tablets.
Very frustrating, it basically means annotations are not available. In my case I like to add margin notes with some details after I figure some part of a paper, this takes a bit of time to type in, and it’s horrible to lose it. It would be kinder to people’s time to just disable the feature until it works.
This is not meant to criticize Paperpile overall, I really like every other aspect of paperpile.