I have set the value of the 'Focal length' field to 12.5. The Metadata Editor shows this as '12.1150':
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Other numbers are similarly affected: entering a value of '35' results in the editor displaying '35.3400':

I've checked the file in a couple of other EXIF viewers, both of which confirm that the expected value of 12.5 has been stored, so this appears to be just a display issue in the editor.
The hoverover on the file shows it correctly:
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I've also noticed that the 'Focal length (35mm)' field won't accept the decimal separator, limiting it to integers - is this intentional?