Cannot open calculator app from keyboard calculator button

I found that installing the calculator app via apt instead of snap (the default in 18.04) fixed this issue:

sudo snap remove gnome-calculator
sudo apt install gnome-calculator

This has the added benefit of making the calculator launch instantaneously again.

The cause of the problem is a bug in the gnome-settings-daemon package, which has been fixed in Ubuntu 18.10 and above. The workaround above remains necessary in 18.04, however.

You can remove the default shortcut for "Launch Calculator"...

and instead add a custom shortcut to run gnome-calculator, and assign the Calculator key to it: