It was a misimplementation of nip10. I build the thread by the first non-mention e tag after the root, but it should have been the last:

http://git.jb55.com/damus/commit/68ed3d7796d954633fe693f27bb2533e89876d48.html

Damus only adds 2 e tags: root and reply. It seems amethyst/plebstr adds root and multiple e tags when replying.