I feel sure it is Oplismenus hirtellus ssp.undulatifolium !
A grass after all then!
I have only met it once, I hope it is not going invasive in the Atlantic Forest!
As the winters are so mild over there, if would even not lose it foliage, spreading much quicker than in areas with cold winters!
Thanks Porkpal for finding it and Janet for your much appreciated in-depth assistance as always!