I searched these Issues page & did not find this:

alexsmobs.toml config file has:

boneSeprentSpawnRolls = 40

I'm guessing it should be:

boneSerpentSpawnRolls = 40
..

... as per:

boneSerpentSpawnWeight = 8

EDIT:
alexsmobs-1.21.1.jar
mc 1.19.2