I just noticed this problem last evening on a computer I moved to a new location. (running RB 2018 -5)

In the previous location I had room for 2 monitors, and I had been running RB on the 2nd monitor. In the new location (where there is only room for one monitor) every time I tried to open RB I could see that it was active (as shown in the task bar) but I couldn't get it to un-minimize, even if I changed the shortcut to "open maximized"

I think something got written to one of the CFG files telling it that it was last opened on monitor #2, because that's the same symptom I get on another computer that DOES still have 2 monitors, but the 2nd one is turned off.

Reinstalling solved the problem, apparently by overwriting the file that had saved the previous session's settings.

I mention this for the sake of anyone who might be trying to troubleshoot the same symptoms in the future.