Hello!

I am developer of the Better Combat mod.
We have the following incompatibility: ZsoltMolnarrr/BetterCombat#156 (comment)

When Better Combat is installed, the entity.damage(DamageSource ...) function is only executed on the server side. But the server side cannot spawn particles.

So the only way to resolve this is to send a packet to the client with the information about particles.
I understand this would require turning your mod not purely clientside.

If this would be implemented your mod could continue to function in clientside only and in server supported modes too.

I found the issue on my side.
I was missing some client side logic.