This has to do with whether or not min
is listed as a "recognised function".
You get the first behavior you list (where min
is subscripted) if it is not listed as a function.
You get the second behaviour if min
is listed as a recognised function. I believe min is listed by default.
Perhaps min
has been "re-listed" as a result of an update or an Office repair.
A button in the Math Autocorrect dialog box leads to a further dialog where you can maintain this list.