That’s interesting—and admittedly, also logical.
I think I can offer a small amendment to this description of the logic (as my table has several hundred rows and it’s still happening):
This conversion happens when the column has no values.
I’m using this column as a UI helper, so unless I’m actively editing a row, no row has a value in this column. That at least explains why I couldn’t immediately replicate the behaviour: I’d left a value in this column on another row. Once I cleared it and tried again, the data type conversion happened “reliably”.
I definitely don’t want the conversion logic on this column, because I’m not trying to enter just a date and have this field treated as a date. (It’s also a little odd that it defaults to the year 2001 instead of the current year.)