When one calls the WolframAlpha API, the response contains an internal wrapper instead of pure MathML. This is a novel phenomenon that hasn't occurred earlier this year. Example:

CalculateUtilities`HTMLMathMLUtilities`Private`toXML[.......]

This occurs in this result:

{
    "title": "Decimal approximation",
    "scanner": "Numeric",
    "id": "DecimalApproximation",
    "position": 299,
    "error": false,
    "numsubpods": 1,
    "primary": true,
    "subpods": [
        {
            "title": "",
            "plaintext": "0.78674913154721406857192161553239091731843262422412088229451313309870531901...",
            "mathml": "<math xmlns='http://www.w3.org/1998/Math/MathML'\n    mathematica:form='StandardForm'\n    xmlns:mathematica='http://www.wolfram.com/XML/'>\n <mtable>\n  <mtr>\n   <mtd>CalculateUtilities`HTMLMathMLUtilities`Private`toXML[0.7867491315472140685719216155323909173184326242241208822945131330987053190 &#8230;]</mtd>\n  </mtr>\n </mtable>\n</math>"
        }
    ],
    "expressiontypes": {
        "name": "Default"
    },
    "states": [
        {
            "name": "More digits",
            "input": "DecimalApproximation__More digits"
        }
    ]
}

Addendum: With powers such as 10^157, the MathML output even becomes some kind of ASCII art, which definitively is a bug:

   <mtd>                                                                                                                                157
CalculateUtilities`HTMLMathMLUtilities`Private`toXML[(9.332621544394415268169923885626670049071596826438162146859296389522 &#8230;) 10   ]</mtd>