A bit difficult to describe, but it’s happened multiple times.
I have a field group with nine fields. The last one is a repeater, containing four fields. After making changes to the repeater fields (eg changing their type of fields 2 and 3 from “text” to “number”) and saving, two new rows appear in the main field group. The rows correspond to the repeater rows just changed (eg 2 and 3 in the example above).
It’s not a huge problem since I can simply delete the incorrectly added rows.