WordPress hasn’t changed the default font in at least 4 years (as far as I could go back on the same file on GitHub, it’s almost certainly much longer than that). It is still Georgia. If it is changing it would be because your theme is loading its own styles in the editor so that it reflects how the content appears on the front end. Have you been changing theme?
It’s also worth noting that the font in the editor is not actually being applied to your content. The font will be determined by your theme.
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