If you snap Np++ to the side of the screen, then open a file using the "Edit with Notepad++" context menu, then np++ will rever to it original position. This does not happen if you open the file from within np++.
This happens also If you add Notepad++ to the "Send To" menu and open files from there.
The same problem occurs when maximizing Notepad++ vertically by double clicking on the vertical resize arrow instead of dragging it.
Bugs #2906706, #3057806, #2953513 and #3372330 are all refering to this problem, so it seems there's quite some demand for this to be fixed. I'd be glad to see this problem solved in the near future. :-)
Fixed by Ede_123
Will be in the next version.