Currently, when you submit a modification by a modal, there is no message to alert you about a concurrent editing.
So, if you add a comment after someone has updated the artifact, all modification will be erased by the person submitting its comment in the modal without any alert.
For the moment, a simple fix to improve it is to block on submit and tell to the user that he needs to reload the modal to update the follow-up.
A better way is to display a pop-up like in artifact view with each modification, but this development requires more resource than a simple bug fix.