We have noticed that creating a followup ticket from a side conversation ignores recipient property.
This causes issues in multi-brand environments where the sender address from the previous ticket is not inherited.
The process is described in this article:
This creates a ticket that looks like this:

When we check the recipient property in the ticket api:
{"recipient": null}
This causes the side conversation in the new ticket to use the account default which is not correct.

Thank you for taking the time to provide us with your feedback. This has been logged for our PM team to review. For others who may be interested in this feature request, please add your support by upvoting this post and/or adding your use case to the comments below. Thank you again!