Panels 2.1.0 - also happened with previous versions, but not as often.
When switching back to the app the zoom changes to full page (no longer zoomed in where I had it). Furthermore, it won’t let me zoom back in by double tapping. When I double tap, instead of zooming in, it zooms out to a thumbnail view of the page in the top-left. To restore normal behavior I have to refresh the page, i.e. either swipe to another page then swipe back, or single-tap into page browse mode then select the page I’m on to reload it.
This is happening every time with Panels 2.1.0. In previous versions it happened only one in a while, but not when app-switching. In those versions it would get stuck in that mode sometimes while when zooming in/out.