The only time I've had something like this happen is if I either long press the home button (which is how you open the Google Assistant), or if my phone is lagging when I tap the home button and it thinks I long pressed it. Beyond those two circumstances, I'm not sure why it would be doing this. I'm on an LG V20 running Nougat, so there may be a programming difference with the Harmony I'm not aware of. If nothing else, you can try to disable the Google app, but then you lose all assistant functions.
If you recently updated the Google app, there may be a new bug in it as well. So you could also try uninstalling the updates, which would revert it back to the original version for your phone. That may or may not work, depending on what the problem is.
In the mean time, you should be able to still make and receive calls. If nothing else, instead of pressing the home button, just keep pressing the back button until you reach the home screen.