The mechanic did something to fix an issue, and you’re looking for advice from strangers on the internet, who aren’t looking at your bike, to verify your work? Seems like a way to give yourself unnecessary anxiety.

Some cassette/freehub combos require a spacer, some do not. It depends on how the cogs are dishes relative to the carrier, and overall width. If your bike is now shifting properly, thank the shop and enjoy the ride!