That could be several things from the plants getting too cold, to bugs, to a nutrient lockout caused by pH being off or toxic chemical buildup.
How long has that been happening? is it just a few leaves sporadically occurring around the bottom of the plants, or is it in an area of the plants (upper or lower or middle)? What soil, nutrients, and pH are you using?
There are a variety of leaf defects on the plants. Not all the same and not all over, but consistent enough to draw attention to themselves.
Both older leaves and new leaves look affected.
I used canna terra pro and canna terra nutes and additives always mixed and added at around pH 6.3. My tap water is pH 7.4 but the nutes bring it down to 6.3 on their own without any pH down. On watering days, I use pH down to bring the water down to 6.2-6.5
Two of the plants don't seem affected at all but there are 5 varieties in the grow tent and the rest do show assorted types of leaf defects.