As per https://bugs.mojang.com/browse/MC-84441 the grass path acts similar to farmland with this behaviour.
However, when I stand on farmland, a light level is still shown.
I appreciate that this is functioning as intended mechanically, however shouldn't the diag output from F3 still show a light level to be consistent with other blocks?
Hmm, so it's this bug https://bugs.mojang.com/browse/MC-46667 ?