I believe I've discovered the reason for the "no attributes" error in logs. I'm guessing the deer need a getAttributes method defined inside an AttributeContainer. Here's a quick example from biome makeover.

Hey, thanks for looking into this, and sorry I took so long to get back to you!

I gave it a shot and, unfortunately, that doesn't seem to be the problem - creating an attribute container and returning it doesn't change much.

    companion object {
        // ...
        val ATTRIBUTES = AttributeContainer(
            createMobAttributes()
                .add(EntityAttributes.GENERIC_MAX_HEALTH, 8.0)
                .add(EntityAttributes.GENERIC_MOVEMENT_SPEED, 0.16)
                .build()
        )
    }

	override fun getAttributes(): AttributeContainer = ATTRIBUTES

It was worth a go, though. I'll leave this issue open in case we figure it out.