The predefined category keys are defined as constants in WellKnownTagProps as CAT_*. Only those will get an icon in the tag editor tree. Additional strings are defined in bundles by 3rd party modules, and my toolkit supplies a couple. Any other keys used for tag prop categories display with question marks.
Raw localized strings (key-less) crash the tag editor. Blank tree, blank list of props, no error thrown. 
Raw localized strings are fine for display name and description. (And the only practical way to provide those outside of a 3rd party module.)
Wacky 
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Automation Professionals is pleased to announce new Production releases of this module, with bug fixes to address anomalies in the designer tag browser, discussed over here:
For Ignition v8.3.x: v2.1.6.262181311
For Ignition v8.1.x: v2.1.6.262171502
The tag browser issues do not appear to be other than cosmetic, so upgrading is not urgent.
Edit: v8.3 replaced with minor fix to @runInGateway permissions.
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