Go had the same behavior until recently. Closures captures the variable from the for loop and it was a reference to the value.
They changed it because it's "common" in Go to loop over something and run a goroutine that uses the variable defined in the loop. Workaround was to either shadow the variable with itself before the loop, or to pass the value as an argument.
It's been a long time since I wrote c# so idk if the same is expected from the avg dev, but in Go it's really not explicit that the variable will be a reference instead of a plain value