If you look at the crank arm around the bolt there should be a torque setting suggested. Getting to this torque is best done with a (torque) wrench 8-12" long with a socket adapter.
Since the crank is only 7" long this almost requires a second person to hold the opposite side crank down to get to the suggested torque. I suspect cranks were tightened
"tight enough" but no where near suggested torque. You certainly can't do it with a portable tool kit or even an allen wrench which in 8mm size is ~5 inches long. Riding
a slightly loose crank will very rapidly tear up the splines and ruin the crank.
IIRC the torque for my crankarm was 40nm which is a lot relatively speaking.