Theory time: Neville’s boggart isn’t actually Snape.
Now I know what you’re thinking. “What do you mean it isn’t Snape? It’s in the books and the movies and yada yada yada”. Cool off and let me explain.
Boggarts represent what we fear the most, correct? But it doesn’t necessarily have to be literal. Harry’s boggart was a dementor, but it wasn’t the dementor itself Harry feared, but the feelings they inspired in him. Helplessness, weakness, and the inability to save/help his mother. Harry is afraid of that feeling of weakness, not the dementor itself, but the way it makes him feel.
Same with Hermione. In a post I can’t find but I’m sure you’ve all seen, someone explains that Hermione’s boggart of “failing everything” and "being expelled" is representative of Hermione’s fear of being kicked out of the wizarding world, of rejection. Not necessarily failing, though I’m sure that’s part of it, but the feeling of being rejected and pushed away.
Now, Neville. I think his actual boggart is the feeling of inadequacy. Think. From what we’ve heard about Neville’s life, Neville’s been singled out as inadequate his entire life and disastrous consequences because of it. His Great-Uncle Albie or whatever his name was thought his magic was inadequate, and pushed him off a pier and dangled and dropped him outside a window and who knows what else. His grandmother is constantly holding him against his parent’s standard of character, making him feel lacking. I seem to recall Neville not wanting the boggart to turn into his grandmother, either. And just before the lesson began, Snape made a remark that made Neville feel inadequate to be practicing magic, causing the boggart to don the guise of Prof. Snape.
I bet you anything that if Neville encountered a boggart on the summer holidays, it would take the form of his grandmother or great-uncle or the last person that made him feel like not enough.