Tuple<> is a reference type (class), while KeyValuePair<> is a value type (struct). Using of struct could be useful to decrease memory traffic. If this metric is not valuable in you case you can use Tuple<> as well.

Why using KeyValuePari when you can use Tuple<> ?

Thank you for actually reading the code and your feedback! That behaviour is... something I really wouldn't expect

According to https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2?view=netframework-4.8 :
"For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair<TKey,TValue> structure representing a value and its key. The order in which the items are returned is undefined.", so formattedValues could be in order different that the values were added to the dictionary.