Oh! I understand the issue now, but I'm not 100% sure on the solution just yet.
I'll be able to take a look at this this weekend, but when pressing the "change text" button, before updating STM, please try calling "Canvas.ForceUpdateCanvases()" .
I'm doing a bit more research into this, and it seems that with nested UI elements, the parent element should be set before the child one... so calling layoutGroup.SetLayoutHorizontal(); or layoutGroup.SetLayoutVertical(); before setting STM might also do it. If that works, I'll try to make it so STM checks for this itself when being rebuilt.
I'll keep thinking about the issue, I really want to solve this!