Is "leat bia" grammatically incorrect. "Maith" means good, and it is used a lot in Irish. Like in this example, "it's good you like food" is used instead of simply saying "you like food". Does it have a cultural aspect to it? It'd be interesting if fellow native Irish students would share some stories with us :)
"leat bia" means "with-you food". "Is maith liom... ." is more like "Is good with-me.... ."
So when something is liked, in Irish people say that it "is good with them". It is really a fun idiomatic way of saying things. Irish has a lot of these. Like saying you want something "Tá bia uaim." "Is food from-me". (Or you could use the ba mhaith liom bia expression to be softer I think...).
So if you are saying that the phrase "is maith leat bia" means "it is good you like food" then that is not right. It just means "You like food." or "Is good with-you food" literally.