Another technical problem of Houdini 16 I encountered…
I began to suspect if there is some problem with my hardware…

Render Region window does not match with viewport. It is off as shown on the attached image.

I am working on
Houdini FX (apprentice version)
build 16.0.633
Lenovo Thinkstation P500
Intel Xeon E5-1630V3 @3.7GHz
nVidia Quadro M4000
32Gb RAM

Looks like render region might have got confused when you switched from 2up to 1up. Here's some info on how to log bugs. Please include a step by step explanation on how this catastrophe occurred along with your machine and OS specs. Thanks!
https://www.sidefx.com/forum/topic/15603/ [www.sidefx.com]

It may also be the apprentice rendering resolution limit, especially if you're using a High-DPI monitor.

Ah-ha! I think that might be the reason