User Mention inside macro content is removed when inserted in empty lines

Steps to reproduce:

• go to the sandbox start page (or any other page containing e.g. an "info" macro
• press 'e' to edit - the inplace editor appears
• click into the "info" macro and press "Enter" to add a new line
• in the new line, press '@' to create user mention, and select a user

Expected behavior:

• the mention for the user gets inserted

Observed behavior:

• for a short time a Macro:mention is displayed, then it gets removed. No user mention is inserted.

Inserting the mention in a new, empty line seems to be important for the issue the be reproducible. Doing the same on a non-empty line (even by typing in a simple space) prevents the behavior.

It is not relevant if the in-place editor is used or a dedicated edit mode. It is also reproducible when editing e.g. blog posts, also in the object editor.

What seems to be important is that the mention is inserted inside a macro; e.g. inserting the '@' inside a "info box" as selected from the "Styles" menu does not trigger the issue, as that option only creates a div with a class="box infomessage", and no macro.