Weird behaviour when using read_line in a loop

My first program in Rust is supposed to take input from the user in the form of characters, either C or F:

use std::io;

fn main() {
    let mut srcunit = String::new();

    let mut switch = true;
    while switch {
        println!("source unit? F or C?");
        io::stdin().read_line(&mut srcunit).expect(
            "failed to read src unit",
        );

        if srcunit.trim() == "F" || srcunit.trim() == "C" {
            println!("doing things right with {}", srcunit);
            switch = false;
        } else {
            println!("either F or C, not {}", srcunit);
        }
    }

    println!("you pressed {}", srcunit);
}

When I start the program and press either F or C, it works right, so I'm skipping this here. The weird part comes when I press another character. I expect my program to ask again for either F or C until I press one of those characters. When I do, it should leave the while loop and tell me what I've pressed.

source unit? F or C?
G
either F or C, not G //so far so good

source unit? F or C?
F
either F or C, not G //why is F not assigned to the srcunit variable? It's supposed to leave the loop now.
F                    //why does it print this line? I didn't press a key or have a println function/macro that does this

source unit? F or C?
V
either F or C, not G //it's still G, wtf
F                    //again, why are those two lines printed? Where does it store, that I pressed F previously?
V