I am not sure. It seems like a pretty dumb error to make. To be fair, I have made projects with multi cells on a single charger but this was before the multi-cell boards became common. Also, in those cases I used 18650s in a caddy as they are much less likely to cause a problem when a cell degrades (and you can replace a cell.) Here is the answer to one per cell:

And this is how I do the 18650 packs now.

(I was also in a hurry and needed a dozen packs in 3 days and knew they would only be worn for 10 days max before being handed back to me.)