This looks like a bug, and there is a closed thread that is very similar
Essentially I have a repeating group that is populated from a state. The state is defined on the page load and then items can be added to it on the page. However, when I try to delete an item from the repeating group, it takes out the item but keeps it as a blank one. So length does not reduce, but it just keeps it as an empty object.
I tried to minus one from the state, but that actually did nothing at all.
My workaround is that the repeating group is filtered to show non empty items.
I have a feeling this is state related, as I have never encountered such issues with Things.