Behaviour of 'One-time Cc' field

Has anyone had the following problem? (RT 5.0.3 on Debian 12.)

When replying to a ticket, the top field is ‘One-time Cc’. Choose an email address from the tick-boxes. That address is then added to the One-time Cc field. The address is also visible at the bottom of the page, under ‘On Correspond Notify Other Recipients’.

Next, choose an article from the ‘Include Article’ menu. As expected, the article text is added to the reply field. However, the user we ticked at step one is removed from the One-time Cc field and from the ‘On Correspond Notify Other Recipients’ section at the bottom. The tick-box against the address remains ticked.

When ‘Update Ticket’ is clicked, the reply is not sent to the ticked address.