@Ondřej-Šutera ok new update. It has nothing to do with opening pages or changing models. It happens consistently after each "swipe back" gesture. If a swipe back is done at any time, in any point of the app, the timeline permanently disappears (until the next restart).
I guess this also explains these inconsistent reports: swiping back is not the only way to exit pages and menu. Sometimes we go back tapping on the X, or directly on the map. In those cases, the timeline stays in place.