Slightly unsymmetric stiffness has been a longstanding bug in the game. The two wings deform differently under load (twist and bend), causing the difference in lift and a roll (usually to the left in my experience, but that’s not a certain thing). It’s also the source of the imfamous “why does my plane veer to the left on the runway?” behavior. Stiffer structure (struts, autostrut, KJR) helps, but you can’t get rid of it completely.
For clarification, the game only simulates objects that are the active vessel or within ~2 km of it. Everything else is "on rails," meaning it only moves along Keplerian conic orbits, just as if you engage standard time warp. This reduces processor load.
However, it also means only gravitational accelerations apply to these other objects. No engine thrust is obvious, but also no aerodynamics are applied either.
This isn't a problem with airless bodies; the vessel continues on rails forever unless it impacts the surface. Atmospheres, however, are a problem, because drag and lift are significant forces and have to be actively simulated. So the devs decided that an atmosphere pressure less than 1% Kerbin sea level pressure could be ignored under certain conditions. But anything leaving the simulation "bubble" in an atmosphere greater than that (or entering such an atmosphere) is assumed to experience "significant aerodynamic forces," and burn up, or lose control and crash. The game simply deletes the object to save processor effort.
On Kerbin 1% atmosphere is anything below 23 km. By exiting the flight scene before landing, you're telling the game that you don't care what happens to that vessel, and it can safely assume it will be destroyed. Mods such as StageRecovery can override this behavior, so if you want to recover the vessel you need to either install such a mod or ride it all the way down.
pincushionman's post in KSP Steam was marked as the answer
Click the "x of y downloads completed" on the bottom of the screen to bring up the download manager, and make sure there isn't something else holding it up.
I'd recommend recording in 1280x720 in windowed mode, myself, which is both 16:9 and a "standard" HD video resolution. It also means "bigger" GUI elements, which is useful for Youtube viewers who watch in non-fullscreen mode or on smartphones. And in windowed mode you have quick access to other stuff like capture software contols and whatnot.
pincushionman's post in Symmetric placement not really symmetric? was marked as the answer