Looks possibly like a nutrient deficiency. When I grow in pots, I add slow-release fertilizer when I plant and then use water-soluble fertilizer every week.

Thanks for the advice; I'll fertilize them some more. (I've been fertilizing and couldn't decide whether I was giving enough food, so having your opinion is very helpful!)

What kind of fertilizer were you using?I know dahlias don't like nitrogen so that could have been why.. did you ever find out what was causing this problem?

Either nitrogen deficiency or a rare sulfate deficiency because mine is doing the same thing right now. Just one plant and it's in a pot as well.

https://youtu.be/qPVb7XnUMX4