This popup was recently introduced in PR-2174.
I don't know exactly why, but voyager prompting me that I'm about to lose my progress was sort of strange to me in the context of scrolling on lemmy.
I understand why someone wants such a feature, but IMO this should be turned off by default and available as an option.
It works only on the "back" event from android navigation, not the "back error" on top of the screen.