OpenXcom dev here
Any chance you can take a look at the windowed mode? I've seen this happen in a number of games in windowed mode as it is native Windows behaviour - each time the game is opened the window position is moved slightly down and to the left, and this can result in the bottom of the window going off the bottom of the screen, underneath the Windows task bar. Additionally, I've then had the entire window get stuck under the screen (on Windows 10) when I've tried to move it via the window title bar with the mouse.
If you could set it so that the window position is fixed (say centred on the screen, or maybe positioned by a choice, e.g. top left, bottom left, etc, or even to just store the last used window position in the config file so that it opens up in the same place the next time it is loaded up) then that would be great. The BStone source port for the Blake Stone games appears to centre the window on screen every time, so I never have an issue with that game.