I'm not able to reproduce this. I can confirm one behavior that may seem buggy - if you're in a testplay session, and move the window, this movement will not be remembered next time you open NL. But it should simply go back to the position the window was at last time NL was run normally (ie: not in testplay mode), it shouldn't vary each time. This itself is simply due to that NL does not save settings when in testplay mode, which includes the window position.
Does there seem to be any pattern to it? (eg. is it moving by a certain amount each time, or is it the same position the first time after each "normal" session, etc?)