Yep, that's expected...
I consider this as external issue. Regardless the fact you can't reproduce it in bare conhost, because different API are used. ConEmu is allowed to change console buffer size programmatically only.
@miniksa Just reproduced. And I realized that contents did not disappear. In fact, text foreground color was set to the same value as background.
So, if you try to select text in ConEmu you'll see hidden contents. RealConsole looks like empty one, even after text selection, but Ctrl+C do copy hidden text too.
Did a little bit of digging around. There was an issue MSFT: 11561936 where resizing the console window smaller while in traditional mode (word wrap/resize is set to OFF) would lose some of the color attributes. I fixed that one a few weeks ago and it is on its way out to Insiders.