Several hours later I found a simple “workaround”, you need to make an update to your landscape (not proxy), you can change on the property max LODLevel to another value and voila! you have your grass!
I’m using UE5.1 and grass would not spawn, correct nodes, been trying for days to sort it out and adjusting the Max LODLevel immediately fixed it. The grass spawned and I even adjusted Max LODLevel back to -1 and the grass stayed. Very happy to have found this as couldn’t find any other solution.
This is dumb. It worked, but seriously WTF… Mine was set to -1, I reset it to 0, fixed, back to -1. So, this must have a knock on effect elsewhere. Because I have the same value I started with now.