JK080

What happens to these is the converter will fail and create debris, this debris will damage the pump and create a low pressure condition. The debris ends up in the filter and completely restricts it, this prevents fluid from getting to the pump and it will lose all engagements. The only fix for this is to have it rebuilt or replaced

Sounds like a problem with the input sprag, it may have been installed backwards. The coast clutch comes on in 2nd and low and this supports the sprag and that's why it moves

Need to have it scanned for codes, it most likely has one for the torque converter.