During Section Translation publishing, new sections should be positioned before the first appendix section of the target article (if such appendix sections exist). However, this positioning is not natively supported by the "edit" API action, as this API action only supports replacing a section in a given position, not adding a new section to the position.
For this reason, in order to properly add a section to the desired position, we had to implement our own logic for this functionality. Currently, this logic resides in the client-side of the application, meaning that the contents of the new section that is being published are concatenated with the contents of the first appendix section, and this concatenated HTML string is published to the server.
Unfortunately, this client-side implementation has some side-effects that cannot easily be fixed using the current approach (e.g. T311470). This is why, we need to move this logic from the client to the server side, in order to fix the related issues and increase the stability and the predictability of the publishing functionality, since both the section handling and the publishing will happen in the same environment.