Your original breaker did not have a short time protective function but your new breaker does. A trip at 50 sec looks like it would be from this function. You would need to compare your settings to the trip curve to see how the breaker should behave.
Do you have a recording ammeter so you can see how much current is actually being drawn?
Sometimes generators cannot supply the full amount of current possible from the utility. When this happens they may reduce their voltage, in effect becoming a soft starter.