Indeed in Mx7 it works as it should. So my assumption is that this is a Mx8 bug. File a bug report.
I have replicated this behavior and confirmed that indeed, numbers like 19 are auto-competed to 1919 when you tab out. Any 2-digit number in my browsers (edge, Firefox, and Chrome) auto-filled to 19XX. Three digit numbers filled to 0XXX. So, if you want to seize control of this, you are going to need to create a custom date format.
When I create a custom date format (using the option in the Date Picker Widget from the radio button called ‘Date formar’) I could create a date format that is dd/MM/yy. When I do this, the browser is a little smarter. Any dates that end with 01-59 are auto-set to 20XX. Any year that is 60-99 is 19XX. I guess they figure that’s the best guess.
If you hate both of these, you could create an on-change widget.
Yes, I am able to fix easily, however, the issue is that I don’t want to have to put custom date formats (and definitely not on change microflows) on every date in my entire system (there are quite a lot) if I can avoid it. It wasn’t an issue in 7, so I’m wondering what changed?