My analysis of the situation is as follows:

1. Earlier versions automatically corrected the beginning of numbered list
items either consistently or based on a consistent rule: if the previous
sentence ended in a period, then the numbered list item would begin with a
capital. If the introductory sentence ended in a colon, or the previous list
item ended in a semi-colon, the next list item might not be capitalized.

2. Some users complained that they didn't always want numbered list items
capitalized, and they were wasting keystrokes undoing the correction.

3. Word's developers, in attempting to address this complaint, went too far
and disabled AutoCorrection after *all* numbers followed by periods.

4. Either it didn't occur to them to attack other types of numbered lists,
or they concluded that trying to address all of them (including ones that
use letters) would be too drastic. That is, if you don't capitalize after
a., b., c., then what happens when those letters fall at the end of a
sentence? Same for a), b), c), etc. So the change was implemented unevenly.

This analysis is entirely speculative and may be way off base, but I suspect
that something along those lines is the explanation, and the bottom line is
that there isn't much we can do about it.

--
Suzanne S. Barnhill
Microsoft MVP (Word)