The EXIF specification allows EXIF:BodySerialNumber (what Exiftool calls SerialNumber) to be an arbitrary ASCII string. So LR 9.3 isn't conforming with the spec here.
You can post a bug report in the official Adobe feedback forum, where Adobe wants all bug reports and feature suggestions:
https://feedback.photoshop.com/photoshop_family/categories/photoshop_family_photoshop_lightroom
Be sure to include the first ten lines of the menu command Help > System Info. Provide all the details, rather than linking back to this thread -- experience shows that Adobe developers tend not to follow the links. (Safari users, uncheck the option Safari > Preferences > Privacy > Prevent Cross-Site Tracking or use Chrome or Firefox.)
But to build on Bob's comment, I think it's unlikely Adobe will fix this (unless Nikon starts using serial numbers not containing digits) -- there metadata problems affecting large numbers of users that have gone unfixed for years. You might consider using another standard field, such as EXIF:UserComment or one of the many IPTC fields, or perhaps add a hierarchical keyword.
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