So that's why I'm seeing StringBuilder a lot!

this solution is helpful for newbie

10 * n operations, not good :)

there are a constant number of vowels, so it is also O(1)

your solution have poor performance, each addition will create a new string,
google why StringBuilder appeared

a solution with very poor performance as each addition will create a new string,
because of that problem StringBuilder was invented
I am not saying to use StringBuilder, but your solution is certainly not performant

Did you mix in your post "Expected" and "but was"?
For n = 31 ->
"Expected" result is: "Range: 78731 Average: 6326.47 Median: 1920.00" with a length of 45.
That result is the same in all languages. 66 guys passed the C# kata; I think that if there were errors someone would have seen it.
I tried a few solutions without problem. For n = 31 they give the "Expected" result.