Your solution is correct. In Word’s Equation Editor, using the Accent > Bar structure can cause the dot above a lowercase “i” to disappear. This is related to how Word renders mathematical accents rather than a problem with your keyboard or font.
The easiest workaround is the one you found:
Set the equation input format to LaTeX, type:
\overline{i}
Then press Enter or Space to convert it. This keeps the dot on the “i” while placing the line above it.
You can also try UnicodeMath mode and enter:
\overbar(i)
then press Space to build the equation. Word officially supports overbar expressions in its linear equation format.
Microsoft’s documentation about LaTeX and UnicodeMath equations in Word: https://support.microsoft.com/en-us/word/linear-format-equations-using-unicodemath-and-latex-in-word
For your case, I would continue using \overline{i} since you have already confirmed that it gives the required result.