If I press the calculator button in my keyboard, the gnome calculator app doesn't appear. I have double checked that the shortcut is ok in the settings app keyboard bindings config pane.
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.