I use ElementaryOS 5.1. I first noticed the problem with Liferea, then with Libreoffice. Other applications (like Firefox and Thunderbird and the terminal) work fine.
The problem is: When I open the application (e.g. Liferea) it opens somewhere in the far right where I cannot reach it. I use the windows button + arrow key right to go to the next workspace screen. There is always a last, empty screen. When I reach that and still click Windows button + arrow right, I see that there is another window which shows my application. But I don't know how to reach that.
I do have two screens connected to my tower, but I only use one. I suspected a problem with the second one, so I disabled it in the settings. Now it's disabled and the problem is still there.