It would be beneficial if we had a photo of the canna damage. Where are you located?
This is a very unusual problem that is not at all consistent with typical canna growing and I suggest you look for signs of the armyworm or army cutworm. These pests feed at night and usually hide in ground litter during the day. It is very unusual for these pests to attack cannas but the flower head stem would be the likely choice for them to feed.
If the problem is not associated with the cutworm, you may be dealing with a calcium deficiency or excessive nitrogen producing weak extended neck growth.
Do you know the name of the canna cultivars you are growing? Have you ever had a soil test done? Have you tried to shake the plant and see if the head simply breaks off?
This is an interesting problem and maybe someone else has had this experience with neck breakage.

Regards,
Kent