A drop in ORP after a water change with any salt is normal.
The ORP of freshly mixed salt water is around 220-240 mv. So yes it will take a little time to recover depending on the volume of the water change.
I have no clue why this mysterious new salt mix recovered so quickly. Unless somehow there are permaganates in the salt mix. Don't even know if its possible. Randy or Boomer would know. Boomers out of town for awhile. We'll see what Randy says in the morning.