Hm, so it's when you have a prefab, and then change the text in the scene to be different... When I do that from the state of the .gif above (clicking the the first object, editing text) The text does turn bold, but I'm not seeing any performance issues quite yet, so maybe it's a version thing...?
I did just find and fix another bug that could potentially cause a UI layout to rebuild far too often (or not often enough), so there's a slim chance *that* could be the cause, but I didn't apply it to my test project yet or anything, just another bug I fixed.
The autosplit script needs "STMPagination" in your project, which can be found in the sample scenes - I don't think the script would have compiled without it, but if it's creating just one empty object, it's either set to only create one with one setting or another, or an error is happening and it's stopping. Could I see your inspector? Mine looks like this:
It's really just meant for setting up STMPagination in a "page" format like this, the STMPagination script can be used on its own to have overflow text go to another STM component, like once the objects have been laid out, the script could be deleted. I wasn't able to reproduce an error when adding the component to an STM object on my end, but it's an editor script I just wrote, so I might not be accounting for every use-case quite yet.