Unable to reply to an existing conversation via GUI (but can via CLI)

I have a one-to-one conversation thread that I cannot reply to via the GUI, but which DOES work via the CLI. It's odd, as other conversations work fine in both.

In the GUI, i see the latest messages (including ones I've literally just sent directly from the CLI), but I can't add to the conversation thread from the GUI - enter does not send, for instance.

<total guess> As a potential clue, the ONLY thing that's special about the misbehaving conversation thread is that I remember ending up with two one-to-one threads with the same person as, when I clicked New Chat (poss in an earlier version) and picked this person, with whom I already had a chat going, I was given a whole new conversation thread. (I can't replicate this behaviour in the current OSX client.) Since then, though, things have switched to now only showign one convo with that person (both in CLI and GUI), so it's poss that some conversation-thread merging/killing/squashing behavior has left me with read but not write access to the convo via the GUI, or something similar </total guess>

Keybase GUI Version: 1.0.21-20170413194119+9dea6e1

The report identifies the GUI and CLI as the relevant entry points but names no files or tests. First reproduce the conversation state and compare replying through the GUI with the CLI; done means the affected existing conversation accepts replies in the GUI as well as the CLI.