Using a mouse turns off trackpad

Running 10.12.6

MacBook Pro (13-inch, 2016)

Almost always I use a keyboard, mouse plugged into keyboard, and external monitor. These are all attached to the MacBook using an adapter that plugs into one of the USB-C ports.

But every once in a while I turn to my MacBook Pro to do something. When I try to use the trackpad, the pad is locked and will not click. And none of the tap or swipe gestures work.

I read some posts from 10 years ago where people complained that the "Disable trackpad when mouse is plugged in" option got changed.

But I've been using a similar setup up with older portables up till 2016. But it seems that only this new MacBook Pro has the problem.

Is this behavior something I never noticed till now?