It doesn't happen if you select a specific Start Date, only if you leave the Start Date alone: in today's day view, I tap "+" to create an event, and enter the Subject (or not). I tap on the start date in the Calendar entry screen, which leads me to the date and time screen. I then tap the End Date (only) and choose a date 2 days later. Result: the Start Date also displays the date 2 days later instead of staying on today.
It is most likely the start date selection is the trigger for releasing "one day mode" internally. Try tapping Start date, and tap on today.
Not sure I get your suggestion, unless it is to systematically select both start and end dates, i.e. unnecessarily adding an extra step when the default start date should not move.
When scheduling a multi-day event, tap on start date, and tap it back. Then tap on end date. I know it is two extra taps, but doing this will release the "link".
What type of event do you schedule that begins today and goes for several day, that you do more than once?