EDIT
This ticket is non-valid due to me using modded version of Minecraft. I recreated it in Vanilla and made a new ticket: MC-228207 

Sometimes ice doesn't cover the whole river, but there is a circle of the ice missing. The water is missing too, so I'm not sure if the river is generating wrong, or just the ice is somehow removed.
Several instances of this have occured in newly generated chunks, I am yet to test this in multiple worlds