Flat mode's "Oldest First" option doesn't sort in pure chronological order as one would expect by the name. To achieve that result, you have to choose "Oldest First (Ignore Threads)".
It should be renamed to "Oldest First (Preserve Threads)" or something similar that explains what it does. And then "Oldest First (Ignore Threads)" should be renamed to "Oldest First".
"Newest First" also has the same problem.