Please note: I' filing this bug more for completeness.  It was noticed first on
debian-testing, then Lubuntu, and I modified my opensuse tumbleweed box to
mimick the conditions and it occurs there too, so it'll now be filed upstream.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=916105
https://bugs.launchpad.net/ubuntu/+source/lxqt-panel/+bug/1810075


Setup is a machine with two displays, left is landscape and right is in
portrait.

By adding a panel to the portrait display, the menu doesn't open as expected
just above the panel, but can open on the wrong screen (rare), or usually a
distance above the panel (eg. a test  on opensuse tumbleweed box has it open
closer to top of screen that bottom when panel is at the bottom)

To reproduce - 
1.  use machine with two displays
2.  rotate one display to portrait mode (advanced - left is what I used)
3. add panel to bottom of portrait display, adding menu & other desired widgets
to panel (it doesn't matter which, but menu is required) 
4. click on menu on the second (portrait) display, menu will open & display
well above the panel (even on wrong screen sometimes in test, usually though
just well above)

Note: If you don't see this occur, try switching displays so your portrait is
the landscape & vice-versa. I had to do this on my opensuse box (which doesn't
have have a portrait display).  It was used only for test (and I had to change
the landscape/portrait to the second possible combination before I could get
issue to occur). 

This bug will be filed upstream.