So, the second file you uploaded displays that way because they’re not actual dates - in the file itself, the date-times are represented as strings - so exactly the same as what you’d get from system.dataset.formatDates. I’d also argue that while the behavior changed, the new behavior is strictly better - now dates are represented as ‘true’ date objects from Excel’s perspective, instead of strings.
EDIT: As compared to ‘Sites Alex’; notice the 44216 ‘serial number’ date (How Dates Work in Excel - The Calendar System Explained + Video - Excel Campus)
