It seems to separate at the edge of links. See here.
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I swear I've seen an identical issue in the past, but I can't find it now. Maybe it was even yours?
I was able to reproduce by typing *testing'''testing'''testing in wikitext mode (note the list item), then switching to visual, changing the list to normal text (place cursor at the start and press Backspace), then saving the reply. It may also be reproducible by doing other stuff with lists or tables and then removing them.
This seems to be caused by how generated wrapper paragraphs work in VisualEditor (change https://gerrit.wikimedia.org/r/c/VisualEditor/VisualEditor/+/747875 made me think of this).