I've spotted this on my NL#5 autos which admittedly I've not been giving my full attention. It started at the bottom and is slowly working it's way up, appears to be just on the fan leaves for now. I've never had this issue and looking at the charts it's Mg deficiency but would love to hear some second opinions please. The leaf is much lighter green (close to yellow) than the pic appears to show. If you need more pics let me know. Thanks!