I am trying a new superstition or strategy: never use the “go-to-desktop” button (looks like a square with rounded corners, in center-bottom of the screen). Instead, always switch to another app using the task switcher button (3 vertical bars), and exit-to-desktop from that program if needed. Then be sure to have at least one other program open at all times so it can be switched to.
So far (about 3 weeks) it seems to have worked ok on both my samsung (same model is Joel) and on an Onyx eink. A couple times the annotations appear to be missing when I task-switch back to paperpile, but go to the library list and re-open the particular pdf, and they are there.
Still, I really wish for a way to annotate externally, the most simple being just a feature to import a pdf on android (since “share” already exists). That would allow annotations in other programs, and solve several feature requests besides this one I believe.