Pretty much what the titlte says
GoG tested version : 1.10.2 - 1.5.4 to 1.5.6

Of course. Seems to only break when on a server.

I can confirm this is 100% an issue.

Have you restarted the game after editing the settings?

I can confirm that this works in singleplayer. I cannot confirm if it works in multiplayer though.
Perhaps restarting the server could do the trick after making changes to the config.

Can confirm. Altering the spawn values ranging between 1 and 100 doesn't seem to have effect. I tested this in a snow biome where selkies prevented me from starting the game by overwhelming force/numbers. Really love the mobs, but would like them to be much more rare.

Yes, the spawn rate configs is the main reason we haven't implemented this in our modpack.

Can confirm, in a server/client environment, am getting way too many mobs, configs have no effect. Neither does placing talismans

I recommend using "Custom Mob Spawner"