Excellent answer, thank you!
In fact I'm using the Dateinput option and it behaves as you described. It shows the min date in the input field but in background it returns null in the change event. So that confused me initially and it took me a while to find this "bug".
In my case I have two datetime input Widgets, start and end (of an event). The "end" can be empty (= open end). Your workaround example is not working in my case because if the user empties the "end" input of the datetime Widget manually then it returns null as well. So I can't differ programmatically between the user emptied the end datetime input OR the set date/time is lower than the allowed min value. This should be considered when fixing the bug.
So I probably have to store previously the datetime value (if set) before any change happens to be able to differ before and after any change happens.
No big thing but just to mention it: Your emails I receive after you added an answer in the forum here first show your personal image in a huge way and right of your image (scrolling right) the text content appears (see attached email.png). I'm working on Mac and maybe this problem exists only on Macs email client.