You only have to be down a quart or so before the fluid won’t show on the dipstick. So you could still have reasonably normal trans operation when its that low. It’ll hold what, 10-12 quarts?
I’m not saying it doesn’t happen but I’m not going to worry about a transmission locking up on the highway. In over a million miles, a number of transmission failures, and on high mileage vehicles, I have never experienced it myself. This, seems to me, would be truly catastrophic failure with parts laying on the road I think.
Edit: Sorry 2 cents, didn’t see your comment. There is also a solenoid that locks and unlocks the torque converter that could be sticking. Usually felt when coming to a stop and feels like a standard transmission with the clutch not disengaged. Used to be pretty common on well used GMs. About a $300 or so fix since the trans has to come partly out to get at it.