As far as I know, this is just how Facebook permissions work when it comes to sharing an app's content when an app hasn't requested the publish_actions permission.
Due to how FB permissions work these days, I'm not sure there's a feasible solution other than requesting this additional permission as part of the Facebook website login flow which may not be desirable.
Yes thanks for the reply. It seems odd as my Facebook settings are set to public, yet I am aware that Facebook do things entirely differently to the rest of the planet. I'll look into this and see if I can discover what Facebook are upto.