Hi RobinJo, I think you may have harvested earlier than necessary?
I started them from seed from Experimental Farm Network this year...
https://store.experimentalfarmnetwork.org/products/andys-green-mountain-multiplier-onion
My understanding is that you harvest after the tops have fallen over and turn brown. My tops were still green but mostly fallen over and I just couldn't wait any longer. I will plant some in late October when I plant garlic to see if they will overwinter in zone 4b. I will save the rest to see how well they store and plant the survivors in spring.
Here is a link to another thread with some photos and good info.
https://permies.com/t/382078/alliums/Potato-onion-harvest-breeding
I suspect that whenever you put yours back in the ground that some will make it and you can just keep going. Pretty sure you can just plant any of the individual onions no matter the size.