Wow where are you getting new Swee's from??
Here they have bcome very very rare, since the importban.
I'm still having one male.
The other one was one day healthy, the other day he was dead...
The bird was only 2 years old and seemed to be perfectly healthy. The same bird has had a strange thing when I was on holiday. My sis found him laying on his back. She put him in a small box in the cage with water and food nearby and he became healthy again. Untill two months ago or something, when he suddenly died.
The balancing issue. The birds aren't the most stressed ones, but when replacing them, they can show illness they were before the remove able to hide for you. Possible.
Am I remembering correctly you have had another bird having the same problems with balance???
I should worry, Yellow Bellies are birds who become easily ill.
I've experienced that and look where I'm now, just a single male left

My first birds had a bacterial infection, my second hens a fungual...
With nine birds I would seperate the one with having the balancing problems and take some fresh poops and let it examine on different kinds of diseases. It's worth it in my opinion.
Maybe they have the same attack mine had, I asked my sister and she said that the bird was not able to fly for awhile and was panting a lot.
But all the same, I would worry if it looks like the same thing as your hen had, because she died...
But to come back to my own YB's, later this year I will probably get a new hen from England. I'm looking forward to have a nice pair again.
I am hoping you will take my opinion in acount.