Eviebeanz
I do use the pickers notes for certain items. eg if I order vegetarian sausages I make a note not to provide pork or any kind of meat sausages as a substitute
Yes, Tesco does have a policy of overriding an unticked Allow substitutions box for some items. A picker note can often block that.
Also, if one does select Allow substitutions, unless one specifies ONLY or NOT or something, if a chosen substitute is also not available it can lead to something else arriving.
Pre pandemic, with a chat at the door with the delivery driver, it was easy to say one does not want the chosen substitute, not so easy now, though a driver did telephone me and ask in relation to a recent delivery.
The thing is, if the picker is trying to be helpful but does not have the information, it can lead to unwanted substitution.
For example, one time, some years ago, Alpro unsweetened soya drink was not available and a picker, trying to help, sent dairy milk that had had the lactose removed, perhaps thinking that my choice of the Alpro soya drink was a lactose issue, though in fact it is because I am a vegan.
Though substitution is often very well done, for example gluten-free pasta in one shape may get gluten-free pasta in another shape, but not wheat pasta in the original shape.