Forge: 1.19.2 43.2.8
Ars: v3.13.2

so far unknown how to reproduce, but seen reoccurring on our server

Spark Profiler logs showing issue in Whirlisprig init:

com.hollingsworth.arsnouveau.common.entity.Whirlisprig.()7.29%4
Link to full profiler result https://spark.lucko.me/yA4YFwjjrS

guessing its related to the Constructers calling:
if(!level.isClientSide)
MinecraftForge.EVENT_BUS.register(this);

which seems to be only the case for the Whirlisprig (probably for good reason)

compared with EntityWixie
Call to MinecraftForge.EVENT_BUS.register(this); was removed from EntityWixie constructors with commit "Adds wixie, sylph, bookwyrm functionality" on 08/25/2021
SHA: 350ec74

Edit: removed calls from Whirlisprig for testing, all seems to work just fine

Should be fixed next version (1.19+)