I guess the app is taking over the volume buttons for navigation, like some other apps have that options. Maybe a mistake or an upcoming feature that was enabled too soon. At least it explains why I couldn't change the volume of my headphones while commuting and reading Lemmy yesterday.
It's easier to notice in a thread with more comments, but it seems that using the up/down volume keys is "scrolling" the active comment and bringing up the comment navigation bar for the active comment.