Chat logging issue

Users at one site where I have Spark and Openfire deployed have found what looks like a bug, but maybe not. They report:

We have discovered that there is a certain type of chat taking
place on Spark that is not being archived. If a person is in one of the
group chats, they can click on the same of someone in the group chat,
which starts a “sub chat room” between the two individuals. The “sub
chat room” conversation is not being archived. Can you check to see if
there is any way for us to capture that, so 100% of all conversations
taking place on Spark are archived?

This is on Openfire 3.6.4, Spark 2.58 and the Monitoring Service plugin. Is there anything we’re missing? The ability to either log those chat sesions, or prevent people from being to open them, would work. Thanks.