Only first message displayed... sometimes

Testing MailMan3, installed using the Docker containers 0.1.1 (with HyperKitty 1.1.1):

I have two lists with several discussions in each. The participants/comments stats for all discussions show multiple participants and multiple comments. (Note that the numbers are wrong, as discussed in #149 (closed).)

However, when I open a discussion, some of them only display the first message. Others display all messages, one displays only some, but not all, of the messages. In one case, the discussion showed the top two messages, but when a new message was added, it briefly showed and then it and the second message suddenly disappeared, leaving only the first message. In another case, the discussion only showed the top message, but then, for no apparent reason, the rest of the messages appeared.

Suggestions for how to trace/diagnose the problem would be appreciated.