I think I figured it out. They have to be standing and attacking you while on it.
I had these 3 go back and forth 10x and none of them lit until I corralled them in and forced them to stand and attack me from that position.
So this might be the entire trigger. They must enter an attack state in order to trigger being lit on fire, which would be why a drop of 20 blocks (doesn't trigger and attack area/everything rage) wasn't doing anything, and simply running/walking over them wasn't lighting them on fire. I however will light up like a bonfire if I walk over it.
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I think I figured it out. They have to be standing and attacking you while on it.
I had these 3 go back and forth 10x and now of them lit until I corralled them in and forced them to stand and attack me from that position.
So this might be the entire trigger. They must enter an attack state in order to trigger being lit on fire, which would be why a drop of 20 blocks (doesn't trigger and attack area/everything rage) wasn't doing anything, and simply running/walking over them wasn't lighting them on fire. I however will light up like a bonfire if I walk over it.