The terms we are discussing here are "keyboard shortcuts" and "keybindings". They are useful when searching for help.
xev in a terminal to see if the key works.
If it works, open Settings > Devices > Keyboard for a list of shortcuts. If it says Alt without specifying right or left they should both work. Replace any that specify a side and make sure it only says Alt.
If you have trouble making the settings do that, install dconf-editor
sudo apt install dconf-editor
and launch it from the terminal
Locate the search icon and type
keybinding and look through the paths for what you need. In your example navigate to
If the default value is on it should be set to
If it's not, switch default to off and enter exactly the above as a custom entry.
My Alt shortcuts seem to be the same for each side, so maybe yours got reconfigured somehow.