Where are my millimeters gone?

Good evening,

it is not a big deal, but I am curious. I messed up the metadata of some 600 images with Lightroom, or so it seems. The tag FocalLengthIn35mmFormat got somehow changed and it does not display the units (mm) in exiftool. But otherwise the numerical value is correct. Other EXIF tools show the correct value, too, but they also show the units (mm).

As I said, not a big deal, I am just curious what happened. One can set the value, but only with a number, e.g. =36, not with a string ="36 mm".

Cheers, Mixx