Good catch! Likely part of the reason the breaker popped under less than full load is the heat from the arcing at the bus bar causing the thermal part of the breaker to warm up prematurely.
If it's possible, I would suggest moving the breaker to a different (unused) slot in the breaker box and leaving the spot where it was previously unused. The scorching on the bus bar will not do anything good for contact with the new replacement breaker.
Frankly, if the breaker can't be relocated, I would be tempted to replace the bus bar if reasonably possible.
