Ubuntu – Cannot tile the “Files” window to the left or right side by pressing Super + Left/Right keys very often

Environment:

Hardware: A laptop and a standalone monitor connected;

OS: Ubuntu 18.04

I'm very fond of the function to tile windows by side (https://help.ubuntu.com/stable/ubuntu-help/shell-windows-tiled.html.en), but there are something wrong with it on my computer.

Object: The "Files" window. I tried a lot of applications/windows, the tiling shortcuts work fine on all of them but only the "Files" window.

I open the "Files" window, press the Super + Left keys, nothing happened, and neither does Super + Right but I can maximize and restore the window by pressing Super + Up/Down and I can also switch the windows between my two monitors by pressing Super + Shift + Left/Right.

For other applications/windows, the Super + Left/Right/Up/Down all work fine.

BTW, the related settings in Settings – Devices – Keyboard are all correct.

I tried to turn on/off or change some related settings, nothing worked.

At the end I found one way to make the shortcut keys come into effect by dragging the window by the mouse, move it to the left or right side of the standalone monitor until the semi-transparent tiling indication appears, then I can tile the "Files" window to the left/right side of both the laptop monitor or the standalone monitor by Super shortcut keys.

After I close the "Files" window and reopen it, the problem comes again. Drag to tile, the problem is "fixed" again.

[UPDATE]

I just find another way to "fix" this problem by setting the standalone monitor as the primary display. In this case, I can directly tile the "Files" window by pressing Super+Left/Right normally. But I don't prefer this awkward display settings.