netcdf-c: "Name contains illegal characters" in main branch, but not 4.8.1

Changing from NetCDF 4.7.4 or 4.8.1 to the latest head of main, I get an error about invalid characters.

FATAL from PE  0: NetCDF: Name contains illegal characters: 

netcdf_add_variable:  file: RESTART/fv_core.res.nc variable: xaxis_1

This is the message printed out from the application I’m running (ufs-weather-model) within the library FMS (https://github.com/NOAA-GFDL/FMS)

Using GCC 9.4.0 and macOS 11.6

Guessing it’s related to macOS since it doesn’t show up on Linux, but don’t know what has changed since NetCDF 4.7.4. I’m going to dig a little further, but maybe someone knows of a change within NetCDF that would cause this.

Could it be the slash in RESTART/?

Code also works with NetCDF/4.8.1, so it’s something recent.