It seems that rather than performing the Union, it zoom in on the screen. Current Mac OS and Current LB build
It does work but you need to hold shift as well as the listed hot key…
The un-shifted version of + is the = key. The hotkey isn’t ‘Alt =’, it’s ‘Alt +’. I could change it, but what you’re stating here isn’t a bug, it’s just how keyboards work. 
If you used Alt and the + sign on a numeric keypad, it would work as expected.