Sunflower decapitation!

Hello all!

First time poster, novice gardener here in my hour of need!

I'm growing some American Giant sunflowers and everything has been growing peachy thus far, they have a good spot that gets plenty of sun and are regularly watered, but upon investigation this afternoon I was saddened/shocked/homicidialrage to find that two of my budding flowers have been decapitated!

Basically I have no idea what may have caused this, I can see anything on any other sunflowers that looks like it may have caused it, I will be honest and say I have stopped slug pelleting them because I assumed they were just to big for a slug to worry about now, and they are in the back garden so I can rule out foul play by a child or anything, and as seen by the pictures its quite a clean cut so im assuming its been eaten by some pest.

Anyway any help is greatly appreciated, i don't want to lose any more of them!