I had set an InputfieldDatetime field's setting of "yearRange" to "0:+2" and apparently 0, for the number of years before the current year simply is defined as no limit. (This should work for similar datetime field settings that offer a range.)
The formatting expectation for the jQuery-UI datetime picker component is a string value that always has a negative value, a colon separator, and a positive value, indicated by "-x:+y".
The solution here is simple, but in case anyone else runs into this problem, it just takes the following form instead:
yearRange="-0:+2"
Simply include the negative sign in front of the zero! 🤦♂️