If the json seems to not work then check the MobsPropertiesRandomness.log file in the logs folder for any error. The errors are printed each time you /reload

Examples might be wrong, always check the log for errors and wiki page of objects

If no mobs or preset seem to load, make sure you have them in the right folders (data/<namespace>/mobs_properties_randomness/mobs) and make sure file names are all lowercase with only a-z and underscores in the names. Also make sure the datapack is loading through the /datapack list command

Check if the problem still occurs with MPR only