I had this exact issue with ds1s school years ago. I to pack his lunch box much like you do. He would have fruit and yoghurt, then a sandwich with some sort of snack with it to and a muesli bar and rice crackers.
He also ate 3 weet bix with a banana for brekki. When at home he would never eat thus much so I knew it was enough.

Yet week after week I would get notes saying he had no lunch so they got him some from the canteen and that I had to pay xxxx dollars.
In the end I took his lunch bix to his teacher and showed her. I did this as I had spoken to her already and told her what I pack. Then I had the principal call to express concerns that he keeps not getting lunch. Which I asked didnt they find it odd my other child there had luch why would I only send it with one grrr

Turns out he was throwing most of it away as he inew they would then get him food from the canteen! !