Truck stalls during hill climb

Mine does have the assist

 

What gear are you in? 4wd hi or lo? Shifter position in those gears?

 

Mine doesn't beep. It just holds the hill without rolling back. But that's on a normal street incline. It doesn't work on steep stuff.

 

It's happened in 4hi and 4lo on separate occasions. Hasn't happened in 2wd but I also haven't tried climbing any serious hills in 2wd.

Edit: Shifter position was in S1. This has also happened with and without rear locker engaged.

 

Smoking oil is not that bad.

 

I have that same problem on my 2nd gen tacoma. 2008 4.0L I hate it when it happens because when my truck shuts off, my brake don't work good and I have to push it so hard just to stop the truck but even that it doesn't always work. I always roll backwards‍♂️