mi using mrow: $\underset{}{\overset{A}{\to }}$ and without mrow: $\underset{}{\overset{A}{\to }}$

mn using mrow: $\underset{}{\overset{0}{\to }}$ and without mrow: $\underset{}{\overset{0}{\to }}$

mo using mrow: $\underset{}{\overset{\cong }{\to }}$ and without mrow: $\underset{}{\overset{\cong }{\to }}$

For good measure, not inline:

The latter renders as expected without MathJax in Firefox. Am I missing something?