If someone, or you, manually changed the font INSIDE the Mtext Editor box, you cannot change it from outside the Mtext editor without special tools as noted above.
Same issue if anyone changes anything manually inside the Mtext Editor: it's called an override.
Another way- not perfect but quick- is to match the properties and then explode- the font will update. The problem is that it's no longer Mtext then, just a text..
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