At face value, I don't see why Mendix is making a difference between the different dates either.
Catching data from an API and relying on the date type parsing from Mendix can sometimes be hard.
You don't have any control and it's hard to debug.
That's why I sometimes use to load dates (if they are localized f.e.) in as strings and convert them manually in the Import Mapping.
Maybe that can help?