Hi Colum McAndrew,
I have run through a couple of test in a Jive-X cloud instance, and was able to replicate this behavior. As such I have submitted this as JIVE-75147 with our engineering team. If you would like to track and receive updates on this issue, please submit a case in your customer group here in the Jive Community and the next available member of our support team can assist in associating it with your instance.
Additionally, as a workaround, if you "paste without formatting" (ctrl + shift + v / cmd + shift + v) the behavior was not appearing in my testing.
Thank you Jesse. Will do.