As seen on https://en.wikipedia.beta.wmflabs.org/wiki/T355999 the input $\sin \log_a$ generates
<mi>sin</mi> <mo></mo> <msub> <mi>log</mi> <mo></mo> <!-- This element should not be here--> <mrow data-mjx-texclass="ORD"><mi>b</mi></mrow></msub>
However, the mo should not be generated or generated after the msub element